Y. Marfaing

Electrical activity of nitrogen acceptors in ZnO films grown by metalorganic vapor phase epitaxy

The electrical activity of nitrogen as an acceptor in ZnO has been investigated in two ways. First, nitrogen was introduced by means of diallylamine during metalorganic vapor phase epitaxy (MOVPE) yielding incorporation of nitrogen in the range 1016–1021 cm−3. This led to significant compensation of the natural donors with a minimum electron concentration of 5×1014 cm−3. Second, diffusion of nitrogen was carried out on undoped MOVPE layers under high pressure conditions stemming from the decomposition of NH4NO3. Conversion to p-type conductivity was observed in a systematic way with measured hole concentrations up to 6.5×1017 cm−3.

Undoped high‐resistivity cadmium telluride for nuclear radiation detectors

Starting with highly purified feed material, single crystals of semiinsulating CdTe were grown by the use of the traveling heater method, without external chemical compensation. Evaluation of the crystals by the use of electrical and optical measurements showed decisive improvements in quality. Nuclear radiation detectors prepared by the use of this material resulted in an energy resolution (FWHM), at room temperature, of 4.6 and 8.5 keV for γ rays at energies of 122 and 662 keV, respectively. The energy resolution at 122 keV decreased to 3.8 keV at a temperature of −30°C. No polarization effect was observed.

CdTe growth by “multipass thm” and “sublimation thm”

Abstract Two methods of growth, derived from the classical THM, lead to significant improvements in the perfection of CdTe crystals. A greater purity is obtained with the Multipass THM whereas the modified Sublimation THM reduces the stoichiometric departure. Results on the materials physical characterisation are given which establish the advantages of the methods.

Solution hardening and dislocation density reduction in CdTe crystals by Zn addition

Solution hardening due to the incorporation of Zn in CdTe is studied theoretically by means of a model which starts from the differences between the Cd-Te and Zn-Te bond lengths. The critical resolved shear stress (CRSS) for plastic deformation is calculated for the entire composition range. Plastic deformation experiments performed at room temperature on CdTe, Zn0.04Cd0.96Te and Mn0.1Cd0.9Te crystals confirm that the Zn alloy is harder (by a factor of four) while the Mn alloy does not show any hardening effect. These results are complemented by microhardness measurements which give CRSS values in ZnxCd1-xTe in qualitative agreement with the model. Finally the superior crystalline quality of Zn0.04Cd0.96Te is demonstrated by a lower dislocation density (5x104 cm-2) and a narrower rocking curve width (25 arc sec).

Influence of the mercury vapor pressure on the isothermal growth of HgTe over CdTe

The theory developed in the preceding article is applied to the epitaxial growth of HgTe on a CdTe substrate under isothermal EDRI conditions. In the absence of excess mercury, two different growth conditions can be distinguished. In the high‐temperature case, pure HgTe is present at the surface of the epitaxial layer and the layer thickness is proportional to the square root of the Hg vacancy diffusion coefficient and of the vacancy concentration gradient. At low temperature (<450 °C), a CdHgTe solid solution exists at the surface of the epitaxial layers; its thickness depends linearly on the coefficient of interdiffusion and on the mercury concentration gradient at the surface. Mercury in excess influences the rate of interdiffusion, the evaporation of mercury from the substrate, and therefore the interdiffusion coefficient as well as the layer thickness and the surface composition. It was found that the interdiffusion coefficient D (varying between 0.5 μ2/h for cadmium‐rich alloys and 100 μ2/h for merc...

Deep level structure and compensation mechanism in In-doped CdTe crystals

Abstract Deep Level Transient Spectroscopy (DLTS) and Optical Deep Level Transient Spectroscopy (ODLTS) experiments have been conducted on a series of In-doped CdTe crystals grown by the Bridgman or the travelling heater (THM) methods using Te as the solvent. The THM samples are n -type but strongly compensated. Annealing at 700°C under high Cd vapour pressure leads to a decompensation of the crystals. The electron concentration is then a measure of the donor (In) concentration, which was in the range 3 × 10 16 −1.5 × 10 18 cm −3 . Six and eight electron traps are, respectively detected in the non-annealed and annealed samples at a concentration level 10 2 –10 3 times below the net electron concentration. They cover the energy range 0.2–0.8 eV. Similar traps are found in both types of crystals, the concentration of which increases with In content and after annealing. The presence of In interstitial-type defects is suggested. A main hole trap at 0.12eV is detected by ODLTS in compensated samples with a concentration close to the donor concentration. Low temperature electrical measurements show that this trap is ionized under equilibrium conditions. It appears to be the main compensating acceptor centre. A plausible microscopic structure is In cd V cd . This study shows that In-doped CdTe grown by THM is electrically compensated and that In-containing neutral associates or precipitates play a minor role in the compensation mechanism.

Systematic photoluminescence study of CdxHg1−xTe alloys in a wide composition range

A systematic photoluminescence study of CdxHg1−xTe has been carried out versus solid composition to follow step by step the evolution of the optical radiative mechanisms involved. Starting from Cd-rich samples, the recombination of bound excitons and intrinsic excitons trapped in potential fluctuations related to statistic alloy disorder has been observed over a large range of composition (0.2 < x < 1). The localization energy of excitons has been estimated from the thermal behaviour of luminescence; it exhibits a compositional dependence proportional to the alloy disorder function x(1 − x) and it amounts to about 10 meV for x ≈ 0.5.

Fundamental studies on compensation mechanisms in II–VI compounds

Abstract A review of the compensation mechanisms in II–VI compounds and alloys is presented in the light of the more recent experimental and theoretical works. Compensation due to native defects is first considered and then the effects due to shallow-deep transition of impurity levels are described. Three exemplary situations of doping limitation are finally distinguished with the corresponding prototype systems: compensation by native defects (ZnSe:N), achievement of chemical solubility limit (ZnTe:N), impurity deactivation and compensation related to a shallow-deep transition (ZnTe:Ga, ZnSe:As).

Deep centres for optical processing in CdTe

Abstract The characteristics of defect and impurity centres which introduce near-mid-gap localized levels in CdTe are reviewed. Attention is focused on the 0.8–0.9 eV energy depth range corresponding to the wavelength range of optical telecommunications. The most appropriate centres are deep donors of group IV (tin, germanium) and transition metals (either deep donors (titanium, vanadium) or a deep acceptor (nickel)).

Characterization of photorefractive CdTe:V: high two-wave mixing gain with an optimum low-frequency periodic external electric field

We present two-wave mixing results obtained with CdTe:V photorefractive crystals under an external dc or low-frequency square-shaped periodic electric field. The majority photorefractive carriers are holes at 1.32 and 1.54 μm, whereas electrons dominate slightly at 1.048 μm. At this wavelength an intensity-dependent resonant gain enhancement is observed in the presence of a dc field. The effective trap densities are near 1015 cm−3. The highest gains (to as high as 11 cm−1 for a field amplitude of 15 kV/cm at 1.32 μm) are obtained with a square-shaped periodic field at an optimum frequency F0. The dependence of F0 on the different experimental parameters is presented. A two-level model in which the secondary level has a thermal emission rate much larger than the principal one qualitatively describes this nonstandard frequency behavior.