We report Dirac-Landau polaritons observed by terahertz (THz) magnetoreflectivity spectroscopy, demonstrating strong coupling between cyclotron transitions of two-dimensional Dirac fermions in HgTe quantum wells and optical cavity modes. Under pulsed electrical injection we observe efficient nonlinear electroluminescence, with a strongly out-of-equilibrium polariton distribution dominated by emission from the upper polariton branches. Model analysis of the bias-dependent emission intensity and spectral narrowing indicates a polariton occupancy per mode approaching unity, with a possible contribution from stimulated polariton emission in the spectral region of the upper anticrossing. These results open prospects toward Dirac-Landau polariton condensates and low-threshold, tunable THz polariton lasers based on cyclotron emission.

Semiconductor nanowires (NWs) are believed to play a crucial role for future applications in electronics, spintronics and quantum technologies. A potential candidate is HgTe but its sensitivity to nanofabrication processes restrain its development. A way to circumvent this obstacle is the selective area growth technique. Here, in-plane HgTe nanostructures are grown thanks to selective area molecular beam epitaxy on a semi-insulating CdTe substrate covered with a patterned SiO2mask. The shape of these nanostructures is defined by the in-plane orientation of the mask aperture along the <110>, <11¯0>, or <100> direction, the deposited thickness, and the growth temperature (GT). Several micron long in-plane NWs can be achieved as well as more complex nanostructures such as networks, diamond structures or rings. A good selectivity is achieved with very little parasitic growth on the mask even for a GT as low as 140 °C and growth rate up to 0.5 monolayer per second. For <110> oriented NWs, the center of the nanostructure exhibits a trapezoidal shape with {111}B facets and two grains on the sides, while <11¯0> oriented NWs show {111}A facets with adatoms accumulation on the sides of the top surface. Transmission electron microscopy observations reveal a continuous epitaxial relation between the CdTe substrate and the HgTe NW. Measurements of the resistance with four-point scanning tunneling microscopy indicates a good electrical homogeneity along the main NW axis and a thermally activated transport. This growth method paves the way toward the fabrication of complex HgTe-based nanostructures for electronic transport measurements.

We report on the emission of Terahertz radiation from Landau quantized Dirac electrons in HgTe/CdHgTe based quantum wells. We show that this cyclotron emission is continuously tunable with magnetic field from 500 GHz up to 2.5 THz. Moreover, given the relativistic nature of charge carriers, the cyclotron mass and therefore the emitted frequency are also tunable with the electron density. These results pave the way for the long-awaited Landau laser which could additionally be tunable by a gate voltage with a fixed magnetic field.

HgCdTe Avalanche Photo Diodes (APDs) are developed at CEA/Leti to enable applications that require the detection of information contained in a low number of photons in each spatial and/or temporal bin, such as LiDAR and free space optical communications. The requirements for such detectors are strongly application dependent, which is why both the HgCdTe APD technology and the proximity electronics, used to extract the detected photocurrent, needs to be optimized for each application. The present communication reports results obtained from the development of detectors for high dynamic range LiDAR applications, made within the scope of the H2020 project HOLDON, and high data rate FSO, made in collaboration with Mynaric Lasercom AG. For FSO applications, we have measured 10 GHz bandwidth at unity gain for APDs with 10 μm diameter. At higher APD gain and diameter, the BW is presently limited by carrier transit and by resistance-capacitance product in small and large area APDs, respectively. For LiDAR we have developed APDs with an made of an array of diodes in parallel with a diameter up to 200 μm and large avalanche gain, M<100, that will be hybridized with a dedicated CMOS amplifier. This circuit was designed to enable photon shot noise limited linear detection over a dynamic range of 6 order of magnitude of signal for observation times ranging from ns up to μs. First characterizations made at unity APD gain shows that the HOLDON detector will meet most of the required performance parameters in terms of sensitivity and linear dynamic range.

This LETI/Sofradir/Defir study aims at realizing sub-10 mu m pitch HgCdTe infrared FPAs. To cope with the different diode process issues related to pitch reduction -morphologic realization, short-circuits, FTM optimization- a parametric study was carried out -contact size, passivation properties, doping levels, diode processing conditions-. A wafer-level test campaign was conducted to evaluate the process window. It revealed functional MWIR diodes from 15 mu m to 3 mu m pitch. 7.5 mu m pitch 640x512 and 5 mu m pitch 64x152 FPA were characterized and turned out to be functional.

Multicolor detection capabilities, which bring information on the thermal and chemical composition of the scene, are desirable for advanced infrared (IR) imaging systems. This communication reviews intra and multiband solutions developed at CEA-Leti, from dual-band molecular beam epitaxy grown Mercury Cadmium Telluride (MCT) photodiodes to plasmon-enhanced multicolor IR detectors and backside pixelated filters. Spectral responses, quantum efficiency and detector noise performances, pros and cons regarding global system are discussed in regards to technology maturity, pixel pitch reduction, and affordability. From MWIR-LWIR large band to intra MWIR or LWIR bands peaked detection, results underline the full possibility developed at CEA-Leti.

Topological insulator materials like HgTe exhibit unique electronic properties at their interfaces and so peculiar attention has to be paid concerning the growth optimization. Molecular beam epitaxy of tensile-strained HgTe/CdTe is investigated as a function of the growth temperature. Crystal quality is checked by using high resolution X-rays diffraction. By combining several material characterization techniques such as scanning transmission electronic microscopy, time-of-flight secondary ion mass spectroscopy and X-rays reflectivity, we report sharp interface morphology with nanometer-scale Hg/Cd diffusion lengths.

This paper reports the first implementation in our laboratory of a chemical–mechanical polishing (CMP) process for CdZnTe (CZT) substrates prepared for growth of HgCdTe layers by liquid phase epitaxy and molecular beam epitaxy. The process enables significant reduction of the thickness of the damaged zone induced by the mechanical polishing that must be etched away before epitaxy. Resulting improvements in surface morphology, in terms of waviness and density of point defects, are reported. The chemical state of surfaces polished by CMP was characterized by x-ray photoelectron spectroscopy. The chemical state was highly homogeneous; comparison with a reference surface is reported. End use assessment of this surface processing was compared with that of reference substrates by preparation of focal-plane arrays in the medium-wavelength infrared spectral range, by using epitaxial layers grown on substrates polished by different methods. The electro-optical performance of the detectors, in terms of photovoltaic noise operability, are reported. The results reveal that the state of this CMP surface is at the level of the best commercial substrates.

We present molecular beam epitaxy growth of tensile-strained HgTe/CdTe thin films for realization of three-dimensional (3D) topological insulator structures. The growth temperature is investigated by looking at crystal quality using high-resolution x-ray diffraction, being found to be much lower than the usual surface temperature for low-cadmium-fraction HgCdTe material. The strain status of HgTe is checked as a function of thickness, and the first indication of strain relaxation is found to appear for thickness well below the critical thickness expected for this system. Surface and interface morphology are also investigated, and well-defined interfaces as well as atomically flat surfaces are demonstrated. Finally, transmission electron microscopy is used to image the material structure of a HgCdTe/HgTe/HgCdTe stack suitable for electronic transport experiments.

We take advantage of the zinc distribution of (211)B CdZnTe substrates to probe the lattice-mismatch-induced stress in long-wave infrared HgCdTe layers grown by molecular beam epitaxy. High-resolution x-ray diffraction is used to accurately determine the strain-free lattice parameters of both CdZnTe and HgCdTe, together with the in-plane components of the stress tensor. By using several wafers, the stress evolution is derived over a broad range of lattice mismatch. In particular, stress relaxation is evidenced for mismatch greater than 0.02% and 0.04% for tensile and compressively strained HgCdTe, respectively. In-plane strain anisotropy, expected for the (211) orientation, is only evidenced for the compressive configuration. Strain relaxation is correlated with substrate curvature and rocking-curve peak broadening, providing indirect evidence for plastic relaxation.

High resolution diffraction measurements of the strained lattice unit of HgCdTe and CdZnTe have been performed at temperatures varying from room temperature to 300°C and for different lattice mismatch between substrate and layer. This investigation makes possible the determination of the coefficients of thermal expansion (CTE) and the evolution of the HgCdTe film stress during the thermal cycles. It is found that the CTE is linear with the zinc fraction for CdZnTe while it can be described by a parabolic variation as a function of the cadmium fraction for HgCdTe. The temperature evolution of the stress is found to be dictated by the CTE difference between substrate and layer up to a temperature of 150°C above which the HgCdTe layer partially relaxes. The evolution of the stress with lattice mismatch enables the determination of the onsets for plastic relaxation for both tensile and compressive stress.

We present high-resolution diffraction measurements of the lattice parameters of HgCdTe and CdZnTe. These measurements were performed at various temperatures ranging from room temperature up to 300°C, enabling the determination of the coefficients of thermal expansion (CTE) and the evolution of the HgCdTe film stress during the thermal cycling. It is found that the CTE is linear with the zinc fraction for CdZnTe, while it can be described by a parabolic variation as a function of the cadmium fraction for HgCdTe. The temperature evolution of the stress is found to be dictated by the CTE difference between the substrate and epilayer up to a temperature of 150°C, for which the stress is partially relaxed. For the sample grown on CdTe/Ge, the HgCdTe lattice is found to be fully relaxed at room temperature and the thermoelastic evolution of the stress of HgCdTe is imposed by the coefficient of thermal expansion of the germanium substrate.

Topological insulators can be seen as band-insulators with a conducting surface. The surface carriers are Dirac particles with an energy which increases linearly with momentum. This confers extraordinary transport properties characteristic of Dirac matter, a class of materials which electronic properties are "graphene-like". We show how HgTe, a material known to exhibit 2D spin-Hall effect in thin quantum wells,\cite{Konig2007} can be turned into a textbook example of Dirac matter by opening a strain-gap by exploiting the lattice mismatch on CdTe-based substrates. The evidence for Dirac matter found in transport shows up as a divergent Hall angle at low field when the chemical potential coincides with the Dirac point and from the sign of the quantum correction to the conductivity. The material can be engineered at will and is clean (good mobility) and there is little bulk contributions to the conductivity inside the band-gap.

The high dislocation density (2×107/cm2 for a thickness of 7μm) in CdTe(211)B on Ge(211) has become a roadblock for the technological exploitation of this material. We present a systematic study of in situ and post-growth annealing cycles aimed at reducing it. An etch pit density of 2×106/cm2 was achieved by optimizing the growth conditions and annealing the samples in situ. This finding was corroborated by high-resolution X-ray diffraction, atomic force microscopy, photoluminescence and ellipsometry measurements.

We test an alternative ellipsometry data analysis technique to measure the temperature and submicroscopic surface roughness of CdTe/Ge during growth by molecular beam epitaxy. Such technique is based on a parametric model for the dielectric function and second-order differentiation of the data. It was proposed in order to reduce the effects of roughness and surface contamination, which are detrimental to the run-to-run accuracy of the traditional data analysis. We find that the new technique yields more accurate results than does the traditional analysis in the presence of both microscopic roughness and submonolayers of excess Te at the surface. However, our data indicate that the new method does not improve the run-to-run accuracy of the temperature measurements. We conjecture that this failure is due to the presence of macroscopic roughness on all MBE-grown (2 1 1) CdTe surfaces. We also propose a qualitative method to detect the formation of macroscopic roughness during growth.

Objectives. This study was conducted to compare microleakage of two new dentin bonding agents on freshly extracted teeth, cryopreserved teeth, or teeth stored in water containing 0.5% chloramine at 4 degrees C.Methods. Rectangular Class V cavity preparations were made on the buccal and the lingual surface of wisdom teeth. They were filled with either Scotchbond Multi-Purpose and Z100 (3M Dental Products) or with Gluma 2000 and Pekafill (Bayer Dental). After thermocycling, silver staining penetration was evaluated under a light microscope. SEM examination and EDX analysis were performed to evaluate the microleakage pattern. The results were analyzed by the use of a two-way analysis of variance.Results. Cryopreservation for 13 wk or 12 d refigeration did not produce changes in the amount of microleakage. However; 48 d or longer of refrigeration increased microleakage. There was no correlation between changes in microleakage and storage time. Specimens prepared with both dentin bonding agents exhibited the same microleakage values and the same microleakage pattern.Significance. Refrigeration at 4 degrees C in 0.5% chloramine for 48 d or longer may cause an increase in microleakage. Cryopreservation for 13 wk or short-term refrigeration did not affect the microleakage.