Light-induced ionic polarization in CdZnTe:V semiconductor crystals as a source of giant enhancement of nonlinear effects Sharon Shwartz, 1 Mordechai Segev, 1,2 Shlomo Berger, 3 Emil Zolotoyabko, 2,3 and Uri El-Hanany 4 1 Department of Physics, Technion-Israel Institute of Technology, Haifa 32000, Israel 2 Solid State Institute, Technion-Israel Institute of Technology, Haifa 32000, Israel 3 Department of Materials Engineering, Technion-Israel Institute of Technology, Haifa 32000, Israel 4 43C Gordon Street, Rehovot 76287, Israel Received 23 December 2008; revised manuscript received 27 February 2009; published 6 May 2009 We report on experimental observation of a remarkable light-induced increase in the low-frequency dielectric constant in doped CdZnTe:V CZT:V semiconductor crystals and show that this increase is due to electric dipoles forming under illumination. Our findings provide strong evidence that CZT:V undergoes a photoinduced phase transition at room temperature from a nonpolar to polar phase. A related symmetry-breaking effect is responsible for giant light-induced enhancement of the nonlinear effects the electro-optic effect and electrostriction previously observed in these crystals. DOI: 10.1103/PhysRevB.79.193202 PACS numbers: 71.45.Gm, 71.55.Gs, 72.80.Ng, 77.22.-d Introducing semiconductor crystals into the field of non- linear optics is highly desirable since potential devices could be produced by advanced microelectronics technology. For example, photorefractive semiconductors seem to be promis- ing for fast nonlinear devices since the mobility of charge carriers in them is much higher than in other photorefrac- tives; hence, the dielectric relaxation times are much shorter for a given light intensity. 1 Moreover, the electrical and op- tical properties of semiconductor crystals can be manipulated via deep doping centers. At room temperature, deep centers act as traps for charge carriers; hence, the carriers’ concen- tration is dramatically decreased as compared to undoped semiconductors. 2 This is a desired feature for space-charge- field effects since the density of the ionized traps the source of the field is much larger than the density of the charge carriers; thus, free charge cannot screen the space-charge field. In nonlinear optics, this situation is mainly used for stimulating photorefractive effects, in which nonuniform il- lumination gives rise to local variations in the density of the ionized traps, resulting in a space-charge field, which modi- fies the refractive index via electro-optic effects. 1 However, semiconductors exhibit weak electro-optic effects, as com- pared to oxide materials, such as LiNbO 3 or Sr x Ba 1-x Nb 2 O 6 . 3 Therefore, in order to obtain reasonable electro-optics refractive-index changes in semiconductors, one should ap- ply very large electric fields or, alternatively, manipulate the local space-charge concentration to enhance the internal electric fields within the crystal. 4 Both options are very lim- ited, altogether providing index changes well below 10 -3 , thus hampering potential applications. In a sharp contrast to those methods, we have recently reported on the observation of refractive-index changes in the semiconductor CdZnTe:V CZT:V in excess of 0.01. 5,6 This value is huge for semiconductors—possibly the largest reported in inorganic bulk materials. In further studies, we have found that the index change is accompanied by a re- markable modification of the crystalline lattice parameter up to 10 -3 . 6 The mechanism responsible for these huge effects is still not understood. Clearly, our findings cannot be ex- plained by “conventional” electro-optic and electrostriction effects since the field required to support such large effects 1.5 MV / cm would be much larger than dielectric break- down field for CZT 500 kV / cm. Moreover, we observed nonzero electro-optic and electrostriction coefficients also in geometries in which these effects should be forbidden by the initial cubic symmetry of CZT. Finally, we emphasize that the enhancement of the electrostriction and the electro-optic effects occurs also when the illumination is uniform. In fact, the effects are observed everywhere in the bulk. This is in contrast to conventional photorefractive effects which neces- sitate nonuniform illumination, otherwise, when the illumi- nation is uniform such effects appear only in a thin layer near the electrodes, where charge carriers accumulate. Our experiments suggest that a novel mechanism is re- sponsible for these effects, whose understanding could pave the way to long-anticipated applications. For instance, we presented a proof of concept for efficient all-optical beam steering 5 and proposed using the light-enhanced nonlineari- ties in CZT:V for efficient THz generation, frequency con- version, and large self-phase modulation. Our experiments 6 showed that illumination at subband-gap energies, together with a moderate applied electric field, induces large defor- mations of the cubic unit cell. These deformations could be a source of lattice instability leading to the formation of a po- lar phase. If this is correct, we expect a sharp increase in the dielectric susceptibility as the illumination intensity is in- creased. Here, we present experimental studies on the low- frequency impedance in CZT:V under illumination at a subband-gap energies, which support the above hypothesis. Our Cd 1-x Zn x Te crystals are grown by the modified ver- sion of the horizontal Bridgman technique. 7 The nominal Zn concentration is x = 0.01 and the nominal value of the vana- dium doping is about 10 ppm. The samples used are in a form of the 5  5  2 mm 3 platelets with faces oriented per- pendicular to the crystallographic directions 110, 11 ¯ 0, and 001. We apply a bias electric field along the 001 direction between electrodes separated by 2 mm. We mea- sure the resistance and capacitance of the crystal while it is illuminated with a broad circular beam full with at half maximum FWHM of 6 mm at  = 980 nm wavelength PHYSICAL REVIEW B 79, 193202 2009 1098-0121/2009/7919/1932024 ©2009 The American Physical Society 193202-1