Issues associated with the development and exploitation of infrared (IR) and terahertz (THz) radiation detectors based on a narrow-gap “HgCdTe” semiconductor have been discussed. This mercury–cadmium–telluride (MCT) semiconductor can be applied for two-color detector operation in IR and sub-THz spectral ranges. Two-color uncooled and cooled down to 78 K narrow-gap MCT semiconductor thin layers grown using the liquid phase epitaxy or molecular beam epitaxy methods on high-resistive “CdZnTe” or “GaAs” substrates, with bow-type antennas, have been considered both as sub-THz direct detection bolometers and 3 to 10 μm IR photoconductors. Their room temperature noise equivalent power at the frequency ν≈140 GHz and signal-to-noise ratio at the spectral sensitivity maximum under monochromatic (spectral resolution ∼0.1 μm) globar illumination reached the following values; ∼4.5×10−10 W/Hz1/2 and ∼50, respectively. Aspheric lenses used for obtaining the images in the sub-THz spectral region were designed and manufactured. With these detectors, about 140 and 270 GHz imaging data have been demonstrated.
Development of infrared and sub-terahertz radiation detectors at the same sensitive elements on the base of mercurycadmium- telluride (MCT) is reported. Two-color un-cooled and cooled to 78 K narrow-gap MCT semiconductor thin layers, grown by liquid phase epitaxy or molecular beam epitaxy method on high resistivity CdZnTe or GaAs substrates, with bow-type antennas were considered both as sub-terahertz direct detection bolometers and 3 to 10 μm infrared photoconductors. Their room temperature noise equivalent power (NEP) at frequency ~ 140 GHz and signal-to-noise ratio (S/N) in the spectral sensitivity maximum under the monochromatic (spectral resolution of ~0.1 μm) globar illumination were reached NEP ~4.5*10-10 W/Hz1/2 and S/N~50, respectively.
The Cd1-xZnxTe (x = 0.1) crystals from two different manufacturers were studied by photoconductivity (PC) measurements. The samples 1 and 2 were subjected to chemical etching and irradiation with nanosecond laser pulses. The IR transmittance spectra of the crystals before and after laser irradiation were also monitored. The PC spectrum of the sample 1 had a typical one-band shape while the spectrum of the sample 2 exhibited two bands roughly corresponding to the bandgaps of CdTe and Cd1-xZnxTe that could be attributed to inhomogeneities in the surface region of the crystal. The positions of the maximum and red edge of the PC spectra did not correspond to the component compositions x in the bulk of Cd0.9Zn0.1Te crystals, however chemical polishing etching of the samples in a brominemethanol solution or/and laser irradiation led to this correspondence. Moreover, depending of laser pulse energy density, irradiation of Cd1-xZnxTe crystals resulted in a short-wavelength shift of the PC spectra, transformation of two bands to one in the case of the sample 2, and an increase in the photosensitivity of the semiconductor. The laser processing provided equalization of parameters in the surface and bulk regions.
The temperature dependences of the resistivity of detector-grade semi-insulating CdTe and Cd0.9Zn0.1Te single crystals were
investigated. The investigations have revealed that the thermal activation energy can be higher than Eg/2 at T → 0 K or
considerably less than this value, although the Fermi level is located near the middle of the band gap. It is shown that such
an "anomalous" behavior of the electrical characteristics is explained in detail by the features of the compensation of deep
acceptor levels in the semiconductor band gap. A method based on the electroneutrality equation is proposed for the
determination of the ionization energy and compensation degree of the impurity (defect), which is responsible for the
conductivity of the material. The results extracted with the use of this method lead to the prediction that the inversion of the
conductivity type of the semiconductor under certain conditions can occur as the temperature varies during operation of a
In this report a brief review of the semi-intrinsic conductivity phenomenon in doped CdTe:Cl, CdTe:Ga and Cd1- xZnxTe materials used for room temperature X and (gamma) -ray detectors is discussed. The upper limit of the resistivity is analyzed in a framework of a general three- levels Fermi-statistic model. The role of the residual impurities and impurity-defect interaction as well as segregation of impurity in Te inclusions are discussed. Dependence of the elementary native defects energy formation on the Fermi-level position in CdTe is shown and some reactions between them are taken into consideration for the Fermi-level stabilization near the middle of the band-gap. On the bases of the Fermi-level stabilization phenomenon it is shown that a self-compensation and a maximum doping level in CdTe:Cl, CdTe:Ga and Cd1-xZnxTe depend on the absolute energy of the C (V)-band position. Experimental results of EDAX compositional measurements, photoluminescence are used for illustration of these problems.
In this report a new up to dated view on the compensation mechanism in CdTe bulk crystals doped with Cl in concentration up to 1019 cm-3 is given. This concentration of Cl doping gives a high resistivity material. The chlorine atoms can act as shallow donors being in Te sites or can form complex with cadmium vacancies, so called A-centers. Spatially resolved EDAX mapping of CdTe doped revealed nonstoichiometrical areas distributed over the surface. In these areas there are a high concentration of impurities, where the Cl is located in a very small inclusions, while Na is distributed all over the volume of the big inclusions. After a short-time annealing (4 hours) at low temperature (500 degree(s)C) in Cd atmosphere the areas with deviation from stoichiometry mostly disappeared as well as the inclusions which were present inside. This paper includes a review of the different aspects, which can influence a precise compensation mechanism in a non-doped and Cl doped CdTe.
The wall free growth of CdTe by the Bridgman technique under microgravity was performed. This phenomenon called Detached or Dewetting growth was studied in the flight mission STS- 95. At this mission two CdTe crystals were grown co-doped with vanadium and zinc. Identical samples were grown with different ampoule designs. The effect of Detached or Dewetting growth was observed by analysis of crystal surface and roughness by optical and mechanical methods. All results of the crystals grown under microgravity are compared to the earth grown reference samples. Surface differences can be found between the (mu) g and the 1g samples. The surface roughness measurements demonstrate that the detaching was partially successful.
In this work, manganese segregation in vertical Bridgman- grown GaSb crystals has been investigated for different ampule diameters and several growth conditions. Experimental data of their impurity distribution from atomic absorption spectroscopy and Hall measurements have been obtained and compared with numerical analysis carried out with the finite element commercial program.
Second harmonic generation in Lithium Niobate (LN) thin films has been widely studied. This interest is extended to waveguides obtained by the Liquid Phase Epitaxy (LPE) technique due to the high perfection and crystallinity of the films. However the incorporation of vanadium into the film due to the growth technique is still a problem because of the absorption band of this ion in the visible zone of the spectra. In this work the LN films are obtained by the LPE technique on pure LN singledomain substrates in the horizontal LPE geometry. Several temperatures have been used in order to obtain the best crystalline quality. The starting flux used was LiVO3 80 mol%, with a Li rich melt of LiNbO3.
The bulk growth of periodic poled lithium niobate (PPLN) is made by the off-centered Czochralski technique adding an impurity to the melt. The periodic domain structures are obtained with different impurities such as Er, Yb, Nd, Cr, Fe and Y. The impurity distribution along the bulk PPLN crystals has been studied to understand the formation mechanism of the periodic domain structure. The distribution coefficient of the impurities, the temperature fields and the shape of the solid-liquid interface have been found to play a key role in the PPLN formation. The cooling rate and other growth conditions control the size of the areas where the periodic domain structure appears. It has been found that independently of the impurity added to the melt the dopant concentration is constant along the periodical domain structure, while it has been observed that exists a periodical variation of the Nb concentration which is related one to one with the periodical domain structure.