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1.
The degree of compensation and ionization energy of two-electron DX centers in CdF2: In and CdF2: Ga semiconductors are determined by studying the statistical distribution of electrons localized on impurity levels. The sharp temperature dependence of the concentration of neutral donors observed in CdF2: Ga over the temperature range T = 250–400 K is explained by a high compensation degree, K ≥ 0.996. Thus, all Ga ions introduced into a CdF2 crystal lattice during crystal growth form shallow donor levels. However, the concentration of Ga ions that can form bistable DX centers is rather low and is close to the concentration of electrons injected into the crystal during additive coloring. In CdF2: In crystals, the degree of compensation is smaller than that in CdF2: Ga but is sufficiently high and the number of bistable DX centers is not limited. It is concluded that an extremely narrow impurity band forms in the CdF2: Ga semiconductor. For a total charged-impurity concentration of ~1020 cm?3, the width of the impurity band in CdF2: Ga is not likely to exceed ~0.02 eV. 相似文献
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N. V. Tepliakov A. S. Baimuratov Yu. K. Gun’ko A. V. Baranov A. V. Fedorov I. D. Rukhlenko 《Optics and Spectroscopy》2017,122(1):64-68
The strength of the enantioselective interaction of chiral semiconductor nanocrystals with circularly polarized light can be varied over a wide range, which finds a series of important applications in modern nanophotonics. As a rule, this interaction is relatively weak, because the dimension of nanocrystals is much smaller than the wavelength of the optical radiation, and the optical activity of nanocrystals is rather low. In this work, we show theoretically that, by applying ion doping, one can significantly enhance the optical activity of nanocrystals and to vary its magnitude over a wide range of values and over a wide range of frequencies. We show that, by precisely arranging impurities inside nanocrystals, one can optimize the rotatory strengths of intraband transitions, making them 100 times stronger than typical rotatory strengths of small chiral molecules. 相似文献
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A study is reported of the optical properties of wide-gap, predominantly ionic cadmium fluoride crystals in photo-and thermally
stimulated transformations of metastable indium and gallium centers. An analysis of these properties leads one to a conclusion
of gallium having two metastable states (two types of deep centers). The deep-center binding energies and the barriers separating
the shallow (hydrogenic) and deep centers have been determined for both impurities. Configuration-coordinate diagrams are
developed, and microscopic models for the deep centers are proposed. It is concluded that these centers are identical with
the metastable DX centers in typical semiconducting crystals. Thus cadmium fluoride is the most ionic among the crystals where DX centers have thus far been found. The potential of using such crystals for optical information recording is discussed.
Fiz. Tverd. Tela (St. Petersburg) 39, 2130–2136 (December 1997) 相似文献
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A. E. Angervaks S. A. Dimakov S. I. Kliment’ev A. I. Ryskin A. S. Shcheulin 《Optics and Spectroscopy》2002,93(2):307-314
The diffraction efficiency and the recording and relaxation times of dynamic reflection holograms, recorded in CdF2 crystals with bistable centers are studied experimentally in the temperature range 20–100°C. In the model experiments which measured the quality of the wave reflected from the hologram, the dynamic wavefront distortions are demonstrated to be efficiently compensated using a holographic corrector based on these crystals. CdF2 crystals with bistable centers are likely to be useful in solving problems of correction of laser light wavefront and image correction in observation telescopes with nonideal primary mirrors. 相似文献
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A. E. Angervaks S. A. Dimakov S. I. Kliment’ev A. I. Ryskin A. S. Shcheulin 《Optics and Spectroscopy》2006,101(1):131-135
The wavefront reversal upon degenerate four-wave interaction in a CdF2 crystal with bistable In centers is experimentally investigated using a pulsed ruby laser as a pump source. The reflectance and operating speed of the wavefront-conjugating mirror are measured and the quality of the reflected wave, as well as of the compensation of model phase distortions, is estimated. An operating speed of about 15 ns is obtained for such a mirror with a reflectance of up to 2% at room temperature. Compensation of model large-scale distortions yields a gain in the beam divergence of 20 and a quality of compensation of 1.05. 相似文献
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In III-V and II-VI semiconductors, certain nominally electron-donating impurities do not release electrons but instead form deep electron-traps known as "DX centers." While in these compounds, such traps occur only after the introduction of foreign impurity atoms, we find from first-principles calculations that in ternary I-III-VI2 chalcopyrites like CuInSe2 and CuGaSe2, DX-like centers can develop without the presence of any extrinsic impurities. These intrinsic DX centers are suggested as a cause of the difficulties to maintain high efficiencies in CuInSe2-based thin-film solar-cells when the band gap is increased by addition of Ga. 相似文献
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DX center has been characterized in four GaAs---AlAs superlattices grown by MBE at 580°C. The structures are uniformly Si-doped or selectively Si-doped in the AlAs layers or in the middle of the GaAs layers or on both sides of the interfaces. Deep level transient spectroscopy measurements (DLTS) put in evidence one dominant electron trap, with an activation energy for thermal emission of about 0.42eV for all the superlattices. This defect shows a thermally activated capture cross section and a large concentration except for the case where the only GaAs layers are doped. For the first time, a study of the capture reveals a capture activation energy of 0.36-0.37 eV, which allows us to locate the DX level nearly 60 meV below the conduction miniband. From these results, we show that the observed DX center is related to silicon in the AlAs layers. For the case when the AlAs barriers are not doped, the DX level is due to the Si diffusion from the middle of the wells towards the barriers, the Si atoms having diffused during the growth. 相似文献
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P. M. Mooney 《辐射效应与固体损伤》2013,168(1-2):281-298
Abstract The DX center, the lowest energy state of the donor in AIGaAs with x < 0.22, is responsible for the reduced conductivity as well as the persistent photoconductivity observed in this material at low temperature. Extensive studies of the properties of this deep level in Si-doped AIGaAs are reviewed here. Data are presented showing that the characteristics of the DX center remain essentially unchanged when it is resonant with the conduction band (x < 0.22) and that, independent of other compensation mechanisms, the DX center therefore limits the free carrier concentration in Si-doped GaAs to a maximum of about 2 × 1019 cm?3. Recent measurements suggesting that the lattice relaxation involves the motion of the Si atom from the substitutional site toward an interstitial site are also presented. Evidence for the negative U model, that the DX level is the two electron state of the substitutional donor, is discussed. 相似文献
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P. P. Fedorov I. I. Buchinskaya S. P. Ivanov B. P. Sobolev A. S. Shcheulin A. I. Ryskin 《Optics and Spectroscopy》2002,92(1):125-132
The crystal-chemical processes occurring during the growth of cadmium fluoride crystals doped with bistable gallium and indium impurities and during conversion of the crystals to the semiconductor state are considered. It is found that doping with indium does not strongly affect the optical performance of the crystals, while gallium, due to its low solubility in cadmium fluoride, strongly impairs this performance. It is shown that co-doping of the crystals with highly soluble impurities (yttrium, scandium, and gadolinium fluorides) makes it possible to obtain optically perfect gallium-doped crystals. 相似文献
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A. S. Shcheulin A. E. Angervaks A. I. Ryskin R. A. Linke 《Optics and Spectroscopy》2002,92(1):133-141
The mechanisms of recording and decay of holographic gratings in CdF2 crystals with bistable gallium or indium centers are considered. The analysis of the decay kinetics allows one to determine potentialities and conditions for using these crystals both for static recording of holograms and for holography in real time. This time scale is fairly broad and covers the time range from 1 s to 10 μs or shorter. Energy characteristics of the bistable centers, namely, the binding energies of the deep and shallow levels and heights of the barriers between them, are refined. 相似文献
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A study is reported of the conductivity of CdF2 semiconductor crystals doped by indium and gallium donor impurities and residing in a semi-isolated state. The latter results
from self-compensation of the impurities, in the course of which one half of them creates two-electron DX centers, and the second is ionized. Photo-and thermostimulated depolarization of these crystals has been studied. It was
shown that the observed polarization/depolarization phenomena have a nonlocal nature and are due to the charges present in
these crystals changing their positions. These changes may be formally considered as charge displacement to macroscopic distances
considerably in excess of the interatomic ones. The mechanisms responsible for these phenomena are discussed.
Fiz. Tverd. Tela (St. Petersburg) 41, 1575–1581 (September 1999) 相似文献
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A number of experimental and theoretical studies indicate that DX centers in GaAs, its alloys and other III–V semiconductors have negative U properties. Using far infrared localized vibrational mode (LVM) spectroscopy of Si donors in GaAs under large hydrostatic pressure in a diamond anvil cell we have discovered an LVM of the Si DX center. From the ratio of the LVM absorption lines of SiGa and SiDX and the compensation in our GaAs samples, we show unambiguously that two electrons are trapped when the ionized shallow Si donors transform into negatively charged DX centers, in full agreement with the negative U model.Dedicated to H.-J. Queisser on the occasion of his 60th birthday 相似文献
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S. A. Kazanskiĭ S. A. Klimin A. S. Shcheulin M. V. Narozhnyĭ A. I. Ryskin 《Optics and Spectroscopy》2008,104(6):861-865
The differences in the optical spectra of CdF2:In semiconductors with bistable DX centers (concentrated (CdF2)0.9(InF3)0.1 solid solutions) and “standard” samples with a lower impurity concentration used to record holograms are discussed. In contrast to the standard samples, in which complete decay of two-electron DX states and transfer of electrons to shallow donor levels may occur at low temperatures, long-term irradiation of a (CdF2)0.9(InF3)0.1 solid solution by UV or visible light leads to decay of no more than 20% deep centers. The experimental data and estimates of the statistical distribution of electrons over energy levels in this crystal give the total electron concentration, neutral donor concentration, and concentration of deep two-electron centers to be ~5 × 1018 cm?3, ~9 × 1017 cm?3, and more than 1 × 1020 cm?3, respectively. These estimates show that the majority of impurity ions are located in clusters and can form only deep two-electron states in CdF2 crystals with a high indium content. In this case, In3+ ions in a limited concentration (In3+ (~9 × 1017 cm?3) are statistically distributed in the “unperturbed” CdF2 lattice and, as in low-concentrated samples, form DX centers, which possess both shallow hydrogen-like and deep two-electron states. 相似文献
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The establishment of thermal equilibrium between photoinduced (shallow) and ground (deep) states of bistable DX centers in photochromic crystals CdF2:In and CdF2:Ga, which are used as real-time holographic media, is studied based on the notions of chemical kinetics. Two mechanisms of mutual transformation of shallow and deep centers—the tunnel mechanism and the mechanism with the participation of free charge carriers—are considered. Equations describing the decay of a photoinduced shallow state are obtained. These equations take into account the distribution of electrons between the photoinduced and ground states and the conduction band. Analysis of the experimental kinetic curves of the decay of photoexcited shallow centers makes it possible to determine the activation energies and barrier height for thermally activated processes of mutual transformation of shallow and deep centers. In CdF2:In and CdF2:Ga, this barrier, which determines the decay kinetics of holograms, lies above the bottom of the conduction band by ~10 and ~500 meV, respectively. 相似文献