Double-functional (optical and electrical) interferometer was realized using holographic recording of dynamic gratings in the semiconductor crystals of CdTe: V, CdTe:Ti and ferroelectric-pyroelectric crystal Sn2P2S6 (SPS).
Also we introduce novel holographic single-beam wave-front division interferometer that is compact, do not need stabilization and are well suited for real-world applications.
We will describe optical and electrical effects in photorefractive materials- in semiconductors and semiconductor-ferroelectric crystals.
Double-functional (optical and electrical) interferometer was realized using holographic recording of dynamic
gratings in the semiconductor crystal of CdTe: V. Two mechanisms of holographic phase grating recording is
considered: electro-optic effect (relatively slow, in microsecond) and free-carrier gratings {Drude-Lorentz
nonlinearity, fast response in nanoseconds). Fast optical response, based on Drude-Lorentz nonlinearity (also
called plasma nonlinearity) play essential role in the surface- plasmon resonance phenomena.
For the first time the photo-EMF measurements were carried out for CdTe crystals doped with V atoms as a result of the
photogeneration of carriers from deep impurity centers to the conduction band and the tilted geometry was applied that
allowed two-mentional monitoring of the vibration source. The CdTe:V crystals were excited by a He-Ne laser with
λ=1.15 μm (&barh;ω=1.08 eV) and P=2 mW. The mechanism of appearance of the holographic current in the CdTe:V
crystals (adaptive IR-photodetectors) taking into account of real defect structure was proposed. The frequency
dependence of ac photo-EMF (holographic) current for the CdTe:V crystal was measured. It was shown that a low cutoff
frequency for the laser intensity I=0.2 mW/mm2 equals 6.0 kHz that corresponds to the response time of 26 μsec.
Novel photoelectric techniques of characterization of semiconductor laser and photodetector materials have elaborated. The measurements of photodiffusion current spectra allow to determine not only the photoionization energy of impurity centers and intrinsic defects but also the type of the photogenerated carriers. In this work for the first time we have also developed novel and efficient technique for characterization of nonlinear and laser materials by using a time-resolved photoelectric spectroscopy. This make possible to study the processes of trapping and detrapping of photoinduced electrons. Mentioned above techniques was used for characterization of energy structure of defects and transport of carriers in the photorefractive CdTe:V crystals.
The Ti-doped CdTe semiinsulating crystals of high optical quality were grown by the vertical Bridgman technique. The complex optical, photoelectric and photorefractive measurements of CdTe doped with Ti atoms were carried out. It allowed to our knowledge for the first time to determine the photorefractive characteristics of these crystals. Studies of the optical absorption and photodiffusion current made it possible to determine the nature and energy structure of impurity and intrinsic defects as well as to establish their role in the photorefractive effect. It was shown that the excited 3T1(P) state is in resonance with the conduction band. As a result of it the auto-ionization of electrons to the conduction band under the laser excitation take place. The energy-level diagram both of impurity and intrinsic defects in the CdTe:Ti crystals was constructed. It was shown that titanium dopant have advantage over other dopants and that CdTe:Ti has better characteristics for a potential applications. Obtained parameters: high optical holographic gain coefficient (up to 0.60 cm-1), low background absorption (about 0.1 - 0.2 cm-1), high optical quality and homogeneity, almost electronic type photoconductivity (electron-hole competition factor equals 0.94) show that this material can be effectively used as sensor components for both optical and photoelectric applications in the near infrared region.
The studies of the optical and photoelectric properties of Cd1-xHgxTe:V (x≤0.02, Nv = 1019 cm-3) were carried out. The investigated semiinsulating (ρ = 108 - 109 Ωxcm) crystals were grwn by the vertical Bridgman technique. All obtained samples had n-type conductivity. The measurements of absorption, photoluminescence and photodiffusion spectra allowed us to obtain the information about the impurity centers and intrinsic defects. The nature and the position of their energy levels with respect to the crystal energy band were determined. It was shown that the impurity centers are in the two- and three-ionized states. In the case of V2+ ions excited 4T1(F)- and 4A2(F)-states for Cd1-xHgxTe:V (x =0.018) crystal is in resonance with the conduction band. It was found that for these crystals the photogeneration of electrons from impurity levels are determined both by direct photoionization and autoionization of electrons from excited states to the conduction band. It was found that the photosensitivity region for Cd1-xHgxTe:V crystals is protracted up to 1800 nm. The dynamic of electronic processes with the participation of impurity and intrinsic defects were investigated using a time-resolved photoelectric spectroscopy. It was shown that the electric processes, which determine the photosensitivity region of this crystal is high speed and corresponds to the subnanosecond region.
Studies were carried out of the low-temperature optical and photoelectric properties of Ti-doped CdTe and Cd1-xHgxTe crystals. The absorption is due to intracentral transitions from the main 3A2(3F)-state to excited 3T1(3P)- (1.15 eV) and 3T1(3F)-(0.65 eV) states. It was shown that the maximum of photogalvanic current band corresponds to the energy 1.12 eV. It indicates that the excited 3T1(3P)-state of the Ti2+ ions is in the resonance with the conduction band and is located near the bottom at a distance not more than 50 meV. The analogous structure of the absorption and photogalvanic current spectra appear also the Cd1-xHgxTe:Ti crystals. In this case the long-wave edge of absorption band is some what prolonged. Therefore the use of Cd1-xHgxTe:Ti crystals makes it possible to move the spectral region of their photosensitivity to the long-wave side. On the base of the results of photoluminescence and photogalvanic current spectra it was shown that main structural defects for the investigated crystals are VCd, and also acceptor complexes of type (V2-Cd + D+)-. These defects participate in compensation of charge in the investigated crystals, which are semiinsulating and therefore suitable for carrying out the photorefractive measurements.
Time-resolved photoelectric spectroscopy measurements of photorefractive CdT:V crystals were carried out by using a short light pulse with 9 ns duration from a nitrogen laser 337.1 nm. The light pulse was focused through the semitransparency Au-electrode. The stationary monochromatic illumination of crystals allowed to measure the time-resolved photocurrent, which is caused by the detrapping of electrons photogenerated by the pulse laser excitation. The dependence of intensity of pulse photocurrent at the delay time (formula available in paper), which corresponds to its maximum value, on the energy of additional monochromatic illumination was investigated. In the case, the spectral dependence of pulse photocurrent caused by the detrapping process of electrons in CdTe:V crystals has been measured under the different intensity of the electric field. It was shown that the additional illumination at (formula available in paper)leads to the increasing of photocurrent intensity that is caused by the detrapping processes of electrons from impurity centers and intrinsic defects. Obtained results indicate that CdTe:V crystals are high-sensitive ultrafast photorefractive materials which may be also used for the elaboration of fast photodetectors in the near IR-region.
The present paper is devoted to detailed investigation of the low-temperature optical and photoelectric properties of Cd1-xHgxTe:V (x<EQ0.088) crystals which provided information on the optical quality of these crystals and the nature of their inhomogeneity, and also the nature and the position of the energy levels of deep V2+ and V3+ (single and complex) impurity centers and singly charged complex acceptors. It was shown that the anisotropy of complex impurity centers is determined by the nature of the donor atom and its position in the crystal lattice. It should be noted that such crystals were grown for the first time by the Bridgman method. Transport measurements (T=300K) showed that the samples were highly compensated with a dark resistivity greater than 106(Omega) $CTR cm. It was shown that the anisotropic V2+ centers may be caused by formation of complex impurity (V2+Cd+ XTe) centers, where X are the accidental impurities of group VII atoms positioned at anionic sites. The appearance of rhombohedral V2+ centers may be caused by the formation of the complex (V2+Cd+ XTe) centers whose axis is oriented in one of the <110> direction. In the case of the tetragonal V2+ centers the Z atoms are positioned at cationic sites in one of the equivalent <001> directions. In the case of the Cd1-xHGxTe:V crystals Z may be the Hg Atoms.
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