Vibrational-mechanical properties of the highly-mismatched Cd1−xBexTe semiconductor alloy: experiment and ab initio calculations

Abstract

The emerging CdTe–BeTe semiconductor alloy that exhibits a dramatic mismatch in bond covalency and bond stiffness clarifying its vibrational-mechanical properties is used as a benchmark to test the limits of the percolation model (PM) worked out to explain the complex Raman spectra of the related but less contrasted Zn1−xBex-chalcogenides. The test is done by way of experiment ($$x\le 0.11$$ x ≤ 0.11 ), combining Raman scattering with X-ray diffraction at high pressure, and ab initio calculations ($$x$$ x ~ 0–0.5; $$x$$ x ~1). The (macroscopic) bulk modulus $${B}_{0}$$ B 0 drops below the CdTe value on minor Be incorporation, at variance with a linear $${B}_{0}$$ B 0 versus $$x$$ x increase predicted ab initio, thus hinting at large anharmonic effects in the real crystal. Yet, no anomaly occurs at the (microscopic) bond scale as the regular bimodal PM-type Raman signal predicted ab initio for Be–Te in minority ($$x$$ x ~0, 0.5) is barely detected experimentally. At large Be content ($$x$$ x ~1), the same bimodal signal relaxes all the way down to inversion, an unprecedented case. However, specific pressure dependencies of the regular ($$x$$ x ~0, 0.5) and inverted ($$x$$ x ~1) Be–Te Raman doublets are in line with the predictions of the PM. Hence, the PM applies as such to Cd1−xBexTe without further refinement, albeit in a “relaxed” form. This enhances the model’s validity as a generic descriptor of phonons in alloys.

Taxonomy of high-pressure vibration spectra of zincblende semiconductor alloys – The percolation model applied to Cd(1-x)Zn(x)Te

“…However, scenario ① should be moderated in the case of Cd1-xZnxTe. Indeed, the lower/dominant 𝑇𝑂 𝑍𝑛−𝑇𝑒 𝑍𝑛 does not freeze on crossing the upper/minor 𝑇𝑂 𝑍𝑛−𝑇𝑒 𝐶𝑑 at 𝑥 < 𝑥 𝐶𝑑−𝑇𝑒,𝑝𝑒𝑟𝑐𝑜while the "freezing" was observed in all earlier tested zb-SCA 21,27,28 in a similar context, i.e., Cd1-xBexTe 21 , Zn1-xBexTe 28 , Zn1-xBexSe 27,48 , ZnSe1-xSx 49 . Otherwise, at 5 GPa the Zn-Te ab initio signal would be reduced to 𝑇𝑂 𝑍𝑛−𝑇𝑒 𝐶𝑑 ; therefore, contrary to observation (Fig.…”

“…Hence, the historical MREI/CM-taxonomy has become obsolete. More generally, the PM applied to all tested cubic-SCA with zincblende and diamond structures so far 21 , suggesting its universality.…”

“…High quality and purity Cd1-xZnxTe single crystals covering the composition domain (𝑥=0, 0.11, 0.28, 0.53, 0.87, 0.91) are grown by the Bridgman method 30 . A similar Cd0.89Be0.11Te crystal 21 is used for calibration purpose. All studied alloys crystallize in the zincblende structure at 0 GPa, verified by powder X-ray diffraction measurements.…”

“…1 central panel). So far, the Raman singularity has only been resolved with zb-Zn1-xBexVI (with VI standing for Se or Te) that exhibit an unusually large mismatch in bond-length/stiffness [19][20][21] , the bond-related percolation effects being emphasized in this case 2 . Still regarding resolution, the 1-bond→2-phonon PM-doublet is especially clear for the short bond that generally vibrates at high frequency.…”

“…The short-range mechanical (non-polar) coupling that develops in scenario 2 at large Zn content between 𝑇𝑂 𝑍𝑛−𝑇𝑒 𝑍𝑛 and 𝑇𝑂 𝑍𝑛−𝑇𝑒 𝐶𝑑 when forced to proximity by pressure is compared with the long-range electrical coupling involved between 𝐿𝑂 − and 𝐿𝑂 + at small Zn content. The LO-couplings in zb-SCA has been studied in detail in the literature 31,32 , and we have studied the pressure-induced TO-coupling between the two Be-related PM-submodes of various Zn1-xBex-chalcogenides 21,27,28 . Yet, zb-Cd1-xZnxTe offers the first opportunity to conduct a consistent comparison of the TO and LO couplings within the same zb-SCA on the same basis of two coupled harmonic oscillators.…”