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Crystal boules of ZnTe up to 50 mm in length and 30 mm in diameter were grown by the temperature gradient solvent method. We experimentally confirmed that ZnTe crystal grown using elemental raw materials is superior for THz application, owning to the slight free-carrier absorption and secondary phase associated defects scattering, which is consistent with its higher resistivity and crystallization. However, abundant Te-SP particles enriched at the subgrain boundaries are observed in ZnTe crystals grown by compound powders. Moreover, relative lower resistivity is recognized by fitting the I-V curve, which is mainly contributed by the oxygen.
© 2016 Optical Society of America
1. Introduction
Zinc telluride (ZnTe) has been applied in a wide range of areas including but not limited to photovoltaic cells, green LEDs, mid-infrared lasers and electro-optical modulators [1–4]. Due to its large second-order nonlinear susceptibility and favorable phase-matching conditions with 800 nm light, ZnTe crystal has been proved as an attractive material for producing terahertz (THz) emitter and sensor [5, 6]. However, in that the high melting point (1295 °C), it is still challenging to obtain large size ZnTe bulk crystals using melt growth under stoichiometry. Hence, currently solvent technique is usually employed to grow ZnTe crystal. To avoid the introduction of impurities, homogenous solvent is adopted, like Te-rich [7, 8] or Cd-rich [9] conditions. Considering the nonstoichiometry growth, it is crucial to understand and control the starting raw materials ratio and the compounds synthesis, therefore, to improve the manufacturing yield and drive down the cost.
Nearly dislocations free bulk GaN crystals were obtained from the solutions in supersaturated liquid gallium conditions [10]. Maruyama et al. revealed the bulk ZnSe crystals grown by horizontal traveling solvent using selenium solution [11]. Besides, even high melting point intermetallic materials or oxides could be grown within the proper solvent material [12]. According to the ZnTe phase diagram, the given melting point is determined by the mole ratio of the supersaturated melt Zn: Te. Based on our previous study [13], the ratio of Zn: Te = 3: 7 was used to start the crystal growth by temperature gradient solvent method (TGSM). However, in order to achieve the better single crystal yield and shorter growth period, which are highly desirable factors attribute for the cost-effective bulk crystal production, the pre-growth Te-rich polycrystals preparation should be taken into account. Generally, the Te-rich polycrystals are synthesized from elemental zinc and tellurium. However, it costs long time and has a risk of silica ampoule explosion owning to Zn-Te exothermic reaction. Alternative route, utilizing ZnTe polycrystalline powders mixed with extra Te might solve these problems.
Therefore, the primary work of this paper is to manifest the effect of different raw materials on the performances of the resulting ZnTe crystals applied in THz field. ZnTe ingots grown by TGSM using elemental and compound materials, respectively, were sliced and evaluated. The phase structure and crystalline behaviors were compared using X-ray diffraction (XRD) and Raman spectroscopy. To obtain a further insight on the optical characterization, IR and near bandgap transmission measurements were employed. Finally, the THz responses of <110> ZnTe crystals were measured in a THz time-domain spectroscopy (THz-TDS) system. These assessments are important for developing ZnTe-based bulk material optimization strategies.
2. Experiment details
ZnTe ingots with 30 mm diameter and over 50 mm length were grown by the TGSM method combined with the accelerated crucible rotation technique (ACRT), using elemental and compound materials, named ELE-G and COM-G methods, respectively. For the ELE-G condition, high purity elemental Zn (6N) and Te (6N) raw materials were charged into a carbon-coated quartz ampoule with a ratio of Zn: Te = 3: 7, and sealed under a vacuum of less than 2 × 10−5 Pa. For the COM-G growth, high purity ZnTe (6N) compound powders and excess Te (6N) elemental materials with a ratio of ZnTe: Te = 3: 4 were loaded into a carbon-coated quartz ampoule and sealed under a vacuum of 5 × 10−4 Pa. For the ELE-G, the raw materials suffered a synthesis processing in a rotating furnace over 144 hours before they were cooled to room temperature within 12 hours, more detail in [14]. In case of the COM-G, the starting materials were mixed in the same furnace and held at the maximum temperature (1060 °C) for 24 hours before cooling to room temperature, with a whole duration of ~60 hours. Then the growth of ZnTe was performed in the same modified vertical Bridgman furnace under the same growth conditions and cooling histories, within one single ampoule to reduce the chance of contamination. <110> direction wafers free from twins and grain boundaries with the dimension of 5 × 5 × 2 mm3 were cut from the as-grown ZnTe ingots. Then the samples were mechanically lapped and chemo-mechanical polished prior to performing the measurements.
The phase purity of the Te-rich polycrystals and the resulting ZnTe crystals were tested by X-ray powder diffraction experiments, which conducted on a RICOH D/max2500 powder X-ray diffractometer. Crystallization quality of ZnTe crystal was characterized by Raman spectroscopy measured at room temperature using Renishaw RM1000. Raman experiments were performed in back-scattering geometry illuminated by a 785 nm laser, focused down to a 30 μm spot on the sample. The spectral resolution was 0.5 cm−1. SHIMADZU UV-3150 UV-Visible-NIR and Nicolet Nexus Fourier transform infrared spectrometer were used to extract the transmittance spectra at room temperature. Imaging of Te-rich particles was carried out using an IR transmission microscopy (IRTM) system. THz spectra were obtained using THz-TDS system at room temperature, more details in [15]. A mode-locked Ti: sapphire laser by Coherent Company produced optical pulses with a temporal width below 100 fs and center wavelength of 800 nm at a repetition rate of 82 MHz. The THz pulses were generated using a biased GaAs photoconductive antenna and detected by <110>-oriented ZnTe crystal.
3. Results and discussions
The appearance of the as-grown ZnTe ingots looks similar after removing from the ampoules. Figures 1(a) and 1(b) show a typical ZnTe ingot after rough polish under ambient light and intense incandescent light, respectively. These ingots are constructed from ZnTe single crystal, the transition region composed by Te solution and ZnTe, and the almost pure Te region at the heel (last-to-freeze) part of the ingot. The orange-red ZnTe crystal appears much transparent under intense light, which suggests free from grain boundaries, twins and cracks. No significant Te-rich secondary phase particles were visually resolved, although the crystals grown from Te-solvent. The flat interface between ZnTe crystal and the transition region indicates that the feed velocity well matched the temperature gradient during the crystal growth.
Fig. 1 Typical as-grown ZnTe ingot (a) under ambient light, and (b) under intense incandescent light.
To interpret the effects of different raw materials on the as-grown ZnTe crystal, the phase structure was evaluated firstly. Figures 2(a) and 2(b) show typical powder XRD spectra of the pre-growth ZnTe polycrystalline used to grow the COM-G and the ELE-G respectively, consistent with standard XRD spectra of ZnTe and Te (JCPDS numbers of 15-0746 and 36-1452, respectively). The third phase was not resolved in the mixture achieved by both methods. However, the silica ampoule was often ruptured after raw materials mixing by COM-G, which suggested a higher stress in the Te-rich polycrystals. Since the thermal diffusivity and expansion coefficient of Te are considerably higher than that of ZnTe [16], while Te melting point is significantly lower, a strong stress generates by the contraction variation during cooling down to room temperature. This stress is possibly dependent on the relative mixing degree of ZnTe and Te solvent, which is critical in COM-G condition. However, this issue was solved by quenching the ampoule to room temperature within the furnace after holding the feeds at 1060 °C for 24 h. In terms of the ELE-G growth, this phenomenon is rarely occurred, which reflects the higher mixing homogeneity.
Fig. 2 Powder X-ray diffraction patterns of Te-rich polycrystalline before growth (a) COM-G, and (b) ELE-G.
In terms of the resulting as-grown crystals, almost pure ZnTe phase is exhibited in Fig. 3. The Te-rich secondary phase (SP) particles (precipitation/inclusion) owning to the stoichiometric imbalance were not observed by XRD, which is coincident with the high light transmittance. However, the variation of relative diffraction peak intensities between ELE-G and COM-G is possible associated with the given crystallinity variation.
Fig. 3 Powder X-ray diffraction patterns of as-grown ZnTe crystals grown by COM-G (red line) and ELE-G (black line).
To give a further clarify, Fig. 4(a) and 4(b) shows the normalized Raman spectra from 125~500 cm−1 of the as-grown ZnTe crystals taken at room temperature. Besides the first-order Raman scattering peaks corresponding to the transverse optic (TO) and longitudinal optic (LO) modes at the Г-point [17], in the low-frequency region, the second-order Raman scattering 2TA2(K) is observed as well. This provides additional information on the phonon mode frequencies at critical points near the edge of the first Brillouin zone (BZ). Above the LO(Г) region, a double structure, related to the LO(L) + TA(L) and LO(X) + TA(X) contributions is also observed. The inset in Fig. 4 depicts the enlarged LO peak. The assignments of all peaks are consistent with previous publications [17, 18].
Fig. 4 Room temperature Raman spectra of as-grown ZnTe samples (a) COM-G, (b) ELE-G, inserts are the enlarged LO(Г) peak.
The Raman shift variations according to different vibrational modes for the COM-G and the ELE-G samples are within 0.5 cm−1, as shown in Table 1. However, the relative peak intensities are found to be quite different. Generally, the dominated LO(Г) scattering features are determined by the crystallinity. The full width at half maximum (FWHM) of the LO peak is fitted, as shown at the inset of Fig. 4, with FWHM of 2.71 cm−1 and 2.58 cm−1 for the COM-G and the ELE-G samples, respectively. The corresponding peak intensity (I) to FWHM ratio is 8600 and 28100, which suggests a better crystallographic perfection for the ELE-G ZnTe crystals. Camacho et al. revealed that the broadening of the LO peak of zinc-blende ZnTe is contributed by high stress [17]. The TO feature, being much broader and weaker than the LO peak, is sensitive with the defects induced stress. The intensity ratio of I(TO)/ I(LO) is calculated with the value of 0.07 and 0.35 for the COM-G and the ELE-G samples, which reflects a possibly higher density defects in the COM-G ZnTe crystals. The mixed two-phonon combinations LO(L) + TA(L) and LO(X) + TA(X) are usually degraded by the dislocation related strain [17]. Therefore, more pronounced LO(L) + TA(L) and LO(X) + TA(X) reflect a potentially relative low dislocation density in the ELE-G ZnTe crystals.
Table 1. Raman spectrum peaks and the vibrational modes of ZnTe crystals.
Similar to the XRD observation, Te-SP related Raman scattering peaks were not clearly resolved, as seen in Fig. 4, which is possibly attributed to the large laser spot as comparing to Te-rich particles size. The weak Te related vibrational modes could be annihilated by the 2TA2(K), LO(L) + TA(L) and LO(X) + TA(X). However, the lattice misfits induced by the Te-SP particles could result in both the symmetry deteriorated of the peaks mentioned above and the TO and LO peak broadening. The typical Te-rich SP particles in the as-grown COM-G and ELE-G ZnTe crystal are examined by IRTM imaging, as shown in Fig. 5(a) and 5(b), respectively.
Fig. 5 Typical IR images of as-grown ZnTe crystals grown by (a) COM-G, (b) ELE-G, and the size distribution diagrams of Te-SP particles in (c) COM-G, and (d) ELE-G ZnTe.
For the as-grown COM-G ZnTe crystals, abundant of Te-SP particles enriched at the subgrain boundaries are observed, highlighted by the blue regions in Fig. 5(a). Te-SP sizes show a bi-modal distribution, as illustrated in Fig. 5(c). However, for the as-grown ELE-G ZnTe crystals the Te-SP distributes uniformly throughout the sample, with typical size of 6-30 μm, as seen in Fig. 5 (d). Generally, the size and density variations of Te-SP in ZnTe grown by TGSM are mostly attributed to the deviation from the stoichiometric composition [13, 19]. The morphological instability of the liquid-solid interface was significant due to the constitution supercooling under such Te-rich conditions, in turn the cellular growth enhanced.
The trapped chance of Te-rich droplets could be reduced by using ACRT technology during the crystal growth, resulting in a more uniform Te-SP distribution. However, polycrystalline ZnTe powders were possibly enwrapped by the Te solvent due to the insufficient mixing, which was hard to eliminate by superheating before growth. These non-melting powders acted as nucleating centers, tended to introduce twin, grain and subgrain boundaries. Increasing the superheating temperature could enable to avoid this issue, but was not adopted, as higher superheating usually led higher supercooling, which finally enhanced the morphological instability of the liquid-solid interface. It also suffered from the higher vapor pressure and impurities diffusion from the silica ampoule. Conversely, in the ELE-G ZnTe crystals, these kinds of Te-SP particle enrichments were rarely found, which indicated the preferable crystal quality.
On the other hand, the free electron density in ZnTe crystals which contributed by native point defects and impurities should be taken into account, when acting as THz emitters or detectors [20]. Figure 6(a) shows the representative IR transmission spectra of both COM-G and ELE-G ZnTe wafers in the wave-number region from 500 to 4000 cm−1. The curves appeared flat shape with a mean transmittance of 60%-65%, and 55%-60% for the ELE-G and the COM-G ZnTe wafers, respectively. Generally, the transmittance of intrinsic ZnTe crystals can be written as [21]
where R is the reflectivity, d is the sample thickness, and α is the absorption coefficient. If the absorption in the range of 4000 cm−1~500 cm−1 can be ignored, since the IR photon energy is quite lower than the bandgap, the transmittance T is determined only by the reflectivity R for the 2 mm ZnTe. For the normal incidence, reflectivity is defined as:where n is the relative refractive index. Therefore, n can be calculated based on these two equations mentioned above, which given the value of 2.7~3.0 and 3.0~3.35 for the ELE-G and the COM-G ZnTe, respectively. Marple [22] and Sato et. al. [23] reported that the refractive index of ZnTe is approximately 2.7 at room temperature in the wavelength range of 2.5 μm~20 μm by both simulation and experiment. The resulting n calculated using ELE-G ZnTe transmittance is more satisfied. However, the variation for COM-G ZnTe is possibly attributed to the neglect of absorption. Li et al. [24] and Sen et al. [25] reported free carrier absorption, proportional to the number of free electrons, contributed to the lowering of the IR transmittance at 1500 to 500 cm−1. A relatively lower transmittance in this region indicates a higher free carrier concentration and therefore a lower resistivity for the COM-G ZnTe. This is also recognized by fitting the I-V curves, as illustrated in Fig. 6(b). The resistivity of the COM-G ZnTe is approximately 2~3 times lower than that of the ELE-G ZnTe. Tracing to the growth situations, oxygen might play a critical role in the resulting ZnTe crystals. For COM-G, ZnTe powders are easily oxidized during weighing the raw materials. Besides, owning to their small size (~100 μm), oxygen and water from air tend to be absorbed by the ZnTe powders, and are difficult to evacuate. This is also reflected by a lower vacuum when sealing the ampoule using COM-G.Fig. 6 (a) Typical IR transmittance spectra of ZnTe crystals, COM-G (blue line) and ELE-G (red line) and, (b) the corresponding I-V curves.
The effects of component variations and other impurities were also examined by solid-state UV-Vis-NIR spectroscopy at room temperature, typical transmission spectra near the band-edge region are shown in Fig. 7. The band gap is approximately 2.23 eV for both COM-G and ELE-G ZnTe extracted by fitting the (αhν)2 vs hν plot according to Urbach law [26], α is the absorption coefficient, which is in agreement with [27, 28]. The similar band gap suggests the component variations between COM-G and ELE-G ZnTe crystals are negligible. The relatively sharp slope of the absorption edge suggests the low impurity density in both types of crystals, which is attributed to the high purity raw materials adopted.
Fig. 7 The near bandgap transmission spectra of ZnTe crystals at room temperature, COM-G (blue line) and ELE-G (red line), inter is the plot of (αhν)2 vs hν.
The schematic THz-TDS system is diagrammed in Fig. 8 (a). The THz emission and detection spectra measured via optical rectification and electro-optic effect, respectively, were obtained using the 2 mm <110> ZnTe crystals instead of the commercial PC antenna and ZnTe crystal. The time-domain waveforms are shown in Fig. 8 (b) and (c). Through Fourier transformation, Fig. 8 (d) displays the normalized frequency domain THz waveforms generated by the ELE-G and the COM-G ZnTe. It can be concluded that the THz emission signals have wide frequency distribution, with a pulse width (FWHM) of ~1.5 THz for both kind of crystals. This demonstrated either ZnTe crystals can be used for THz generation. Schall [29] et al. assigned the absorption band in ZnTe at ~1.6 THz to a mixture of LO(X)-LA(X) and LO(L)-LA(L) phonon modes. The relative lower intensity of THz spectra for the COM-G wafers indicated the stronger self-absorption within the crystals as comparing to the ELE-G wafers. In terms of the THz detection spectra, the strongest signal is at 0.59 THz, shown in Fig. 8 (e), which is in agreement with the phase matching condition for EO ZnTe. However, the absorption is more pronounced around 1.4 THz for the COM-G wafers. For the ELE-G wafers, THz spectra exhibited a longer decay beyond the maximum signal with the FWHM of 1.15 THz.
Fig. 8 (a) Schematic diagram of THz-TDS, the normalized time domain THz waveforms via (b) optical rectification emission and (c) electro-optic effect detection, and the corresponding frequency domain THz waveforms (d) emission and (e) detection.
The free-carrier absorption (FCA) [30, 31] and the scattering by the secondary phase particles can reduce the terahertz radiation, further degrading the efficiency of optical rectification and electro-optic effect. Therefore, the lower THz efficiency for the COM-G ZnTe crystals is consistent with its higher conductivity (higher free carrier density) and non-uniformity of the Te-SP particles. Especially for THz detection spectra, significant absorption edge was observed in the COM-G ZnTe and identified that this edge mainly resulted from the dispersive propagation of the THz pulse through the electro-optic crystal.
4. Conclusions
ZnTe crystals were grown by TGSM using elemental (ELE-G) and compound (COM-G) materials, which were subsequently waferized and processed for physical property measurements and THz response testing. It is suggested that there is no significant variations in phase structure and component in both kinds of crystals. However, the crystallization quality of the as-grown ZnTe grown by the COM-G is inferior due to the insufficient mixing as comparing to grown by the ELE-G. This was further confirmed by the broad and weak Raman scattering peaks and subgrain boundaries appeared within the crystals. In addition, the resistivity of COM-G ZnTe is lower than ELE-G ZnTe, which is mostly attributed to the role of oxygen.
From the THz emission and detection spectra, that were obtained using the 2 mm <110> ZnTe crystals, it is demonstrated that ELE-G ZnTe crystal is superior for THz application. This is due to its reduced free-carrier absorption and slight secondary phase associated defects scattering, which is consistent with its higher resistivity and better crystalline quality.
The results of this study thus provide useful insights for understanding and improving the performance of ZnTe bulk crystals grown by melt growth for THz application through raw materials control.
Funding
National Natural Science Foundations of China (Nos. 51202197 and 51372205); 111 Project of China (No. B08040), Natural Science Basic Research Plan in Shaanxi Province of China (2016KJXX-09).
Acknowledgments
We thank Mr Yi Zhang from DAHENG New Epoch Tech, INC, for helping the THz measurement.
References and links
1. J. Fan, L. Ouyang, X. Liu, D. Ding, J. Furdyna, D. Smith, and Y. Zhang, “Growth and material properties of ZnTe on GaAs, InP, InAs and GaSb (001) substrates for electronic and optoelectronic device applications,” J. Cryst. Growth 323(1), 127–131 (2011). [CrossRef]
2. H. Saito, V. Zayets, S. Yamagata, and K. Ando, “Room-temperature ferromagnetism in a II-VI diluted magnetic semiconductor Zn(1-x)Cr(x)Te,” Phys. Rev. Lett. 90(20), 207202 (2003). [CrossRef] [PubMed]
3. R. Page, K. Schaffers, L. DeLoach, F. Patel, S. Payne, W. Krupke, K. Chen, and A. Burger, “Cr2+ doped zinc chalcogenides as efficient widely tunable mid-infrared lasers,” IEEE J. Quantum Electron. 33(4), 609–619 (1997). [CrossRef]
4. M. Ziari, W. Steier, P. Ranon, S. Trivedi, and M. B. Klein, “Photorefractivity in vanadium-doped ZnTe,” Appl. Phys. Lett. 60(9), 1052–1054 (1992). [CrossRef]
5. B. Ferguson and X. C. Zhang, “Materials for terahertz science and technology,” Nat. Mater. 1(1), 26–33 (2002). [CrossRef] [PubMed]
6. T. Kampfrath, K. Tanaka, and K. A. Nelson, “Resonant and nonresonant control over matter and light by intense terahertz transients,” Nat. Photonics 7(9), 680–690 (2013). [CrossRef]
7. T. Asahi and K. Sato, “Growth of large diameter ZnTe single crystals by the double crucible liquid encapsulated pulling method,” Phys. Status Solidi 11(7-8), 1167–1173 (2014). [CrossRef]
8. R. Triboulet, K. Phan Van, and G. Didier, ““Cold travelling heater method,” a novel technique of synthesis, purification and growth of CdTe and ZnTe,” J. Cryst. Growth 101(1-4), 216–220 (1990). [CrossRef]
9. A. Fischer, J. Carides, and J. Dresner, “Preparation and properties of n-type ZnTe,” Solid State Commun. 2(6), 157–159 (1964). [CrossRef]
10. I. Grzegory, “High pressure growth of bulk GaN from solutions in gallium,” J. Phys. Condens. Matter 13(32), 6875–6892 (2001). [CrossRef]
11. K. Maruyama, K. Suto, and J. Nishizawa, “ZnSe horizontal traveling solvent growth using selenium solution,” J. Cryst. Growth 216(1-4), 113–118 (2000). [CrossRef]
12. S. Koohpayeh, “Single crystal growth by the traveling solvent technique: A review,” Prog. Cryst. Growth. Ch. (2016). [CrossRef]
13. Y. Xu, H. Liu, Y. He, R. Yang, L. Luo, and W. Jie, “Research into the electrical property variation of undoped CdTe and ZnTe crystals grown under Te-rich conditions,” J. Alloys Compd. 612, 392–397 (2014). [CrossRef]
14. R. Yang, W. Jie, and H. Liu, “Growth of ZnTe single crystals from Te solution by vertical Bridgman method with ACRT,” J. Cryst. Growth 400, 27–33 (2014). [CrossRef]
15. C. Zhang, B. Jin, J. Chen, P. Wu, and M. Tonouchi, “Noncontact evaluation of nondoped InP wafers by terahertz time-domain spectroscopy,” J. Opt. Soc. Am. B 26(9), A1–A5 (2009). [CrossRef]
16. H. J. Holland and K. Beck, “Thermal Expansion of Zinc Telluride from 0° to 460°C,” J. Appl. Phys. 39(7), 3498 (1968). [CrossRef]
17. J. Camacho, I. Loa, A. Cantarero, and K. Syassen, “Vibrational properties of ZnTe at high pressures,” J. Phys. Condens. Matter 14(4), 739–757 (2002). [CrossRef]
18. J. Camacho, A. Cantarero, I. Hernandez-Calderon, and L. Gonzalez, “Raman spectroscopy and photoluminescence of ZnTe thin films grown on GaAs,” J. Appl. Phys. 92(10), 6014–6018 (2002). [CrossRef]
19. R. Triboulet, K. Phan Van, and G. Didier, “Cold travelling heater method, a novel technique of synthesis, purification and growth of CdTe and ZnTe,” J. Cryst. Growth 101(1-4), 216–220 (1990). [CrossRef]
20. J. Bakker, G. Cho, H. Kurz, Q. Wu, and X. Zhang, “Distortion of terahertz pulses in electro-optic sampling,” J. Opt. Soc. Am. B 15(6), 1795–1801 (1998). [CrossRef]
21. R. Yadava, B. Sundersheshu, M. Anandan, R. Bagai, and W. Borle, “Precipitation in CdTe crystals studied through mie scattering,” J. Electron. Mater. 23(12), 1349–1357 (1994). [CrossRef]
22. D. T. F. Marple, “Refractive Index of ZnSe, ZnTe, and CdTe,” J. Appl. Phys. 35(3), 539–542 (1964). [CrossRef]
23. K. Sato and S. Adachi, “Optical properties of ZnTe,” J. Appl. Phys. 73(2), 926–931 (1993). [CrossRef]
24. Y. Li, Z. Gu, G. Li, and W. Jie, “Infrared Transmission Spectra of Cd1-xZnxTe (x = 0.04) Crystals,” J. Electron. Mater. 33(8), 861–866 (2004). [CrossRef]
25. S. Sen, D. R. Rhiger, C. R. Curtis, M. H. Kalisher, H. L. Hettich, and M. C. Currie, “Infrared absorption behavior in CdZnTe substrates,” J. Electron. Mater. 30(6), 611–618 (2001). [CrossRef]
26. F. Urbach, “The long-wavelength edge of photographic sensitivity and of the electronic absorption of solids,” Phys. Rev. 92(5), 1324 (1953). [CrossRef]
27. K. Sato and S. Adachi, “Optical properties of ZnTe,” J. Appl. Phys. 73(2), 926–931 (1993). [CrossRef]
28. K. D. Patel, G. K. Solanki, J. R. Gandhi, and S. G. Patel, “Growth and characterization of ZnTe crystals grown by physical vapor transport technique,” Chalcogenide Lett. 6(9), 393–398 (2009).
29. M. Schall, M. Walther, and P. Uhd Jepsen, “Fundamental and second-order phonon processes in CdTe and ZnTe,” Phys. Rev. B 64(9), 094301 (2001). [CrossRef]
30. S. Vidal, J. Degert, M. Tondusson, J. Oberle, and E. Freysz, “Impact of dispersion, free carriers, and two-photon absorption on the generation of intense terahertz pulses in ZnTe crystals,” Appl. Phys. Lett. 98(19), 191103 (2011). [CrossRef]
31. S. A. Ku, C. M. Tu, W. C. Chu, C. W. Luo, K. H. Wu, A. Yabushita, C. C. Chi, and T. Kobayashi, “Saturation of the free carrier absorption in ZnTe crystals,” Opt. Express 21(12), 13930–13937 (2013). [CrossRef] [PubMed]
