pubmed.ncbi.nlm.nih.gov

Properties of the exotic metastable ST12 germanium allotrope - PubMed

  • ️Sun Jan 01 2017

Properties of the exotic metastable ST12 germanium allotrope

Zhisheng Zhao et al. Nat Commun. 2017.

Abstract

The optical and electronic properties of semiconducting materials are of great importance to a vast range of contemporary technologies. Diamond-cubic germanium is a well-known semiconductor, although other 'exotic' forms may possess distinct properties. In particular, there is currently no consensus for the band gap and electronic structure of ST12-Ge (tP12, P43212) due to experimental limitations in sample preparation and varying theoretical predictions. Here we report clear experimental and theoretical evidence for the intrinsic properties of ST12-Ge, including the first optical measurements on bulk samples. Phase-pure bulk samples of ST12-Ge were synthesized, and the structure and purity were verified using powder X-ray diffraction, transmission electron microscopy, Raman and wavelength/energy dispersive X-ray spectroscopy. Optical measurements indicate that ST12-Ge is a semiconductor with an indirect band gap of 0.59 eV and a direct optical transition at 0.74 eV, which is in good agreement with electrical transport measurements and our first-principles calculations.

PubMed Disclaimer

Figures

Figure 1
Figure 1. Structure of ST12-Ge.

(a) Experimental PXRD pattern (Cu Kα) of ST12-Ge (black points) and Rietveld refinement (blue line), with difference (red line). wRp-background=5.08%, Rp-background=3.84%, χ2=1.15. The inset shows an image of the recovered ST12-Ge sample polished as a bar (scale bar, 2 mm). (b) TEM image (scale bar, 100 nm) of single crystalline grain extracted from the bulk sample and corresponding electron diffraction patterns along different zone axes (scale bars, 5 nm−1).

Figure 2
Figure 2. Raman spectrum of ST12-Ge compared with DFT calculations.

The calculated peaks are shown as Lorentzian profiles with an arbitrary width.

Figure 3
Figure 3. Electrical conductivity of ST12-Ge.

Temperature dependence of the electrical conductivity for ST12-Ge. Inset shows the activation energy fit to the high-temperature data.

Figure 4
Figure 4. Optical properties of ST12-Ge.

Tauc plots of the Kubelka–Munk absorption derived from diffuse reflectance for ST12-Ge and DC-Ge for (a) indirect allowed transitions and (b) direct allowed transitions. The inset in b shows the absorbance derived from transmission measurements using ∼3 wt% ST12- and DC-Ge in KBr.

Figure 5
Figure 5. Electronic structure of ST12-Ge.

(a) Electronic band structure along high-symmetry lines, (b) electronic density of states, (c) imaginary dielectric function from the Bethe–Salpeter equation (BSE) calculation and (d) a schematic diagram of the valence and conduction bands. It is worth noting that the fundamental indirect gap is not along a high-symmetry direction.

Figure 6
Figure 6. Thermal stability of ST12-Ge.

The plot compares the PXRD patterns of ST12-Ge at room temperature before and after heating at various elevated temperatures for 1 h. After heating at 480 K, the XRD shows a mixture of ST12- and DC-Ge phases; after further heating at 528 K, the sample was fully converted to DC-Ge.

Similar articles

Cited by

References

    1. Bosi M. & Attolini G. Germanium: epitaxy and its applications. Prog. Cryst. Growth Ch. 56, 146–174 (2010).
    1. Kittel C. in Introduction to Solid State Physics 8th edn, 190 (John Wiley & Sons, Inc., 2005).
    1. Mujica A., Rubio A., Muñoz A. & Needs R. J. High-pressure phases of group-IV, III–V, and II–VI compounds. Rev. Mod. Phys. 75, 863–912 (2003).
    1. Menoni C. S., Hu J. Z. & Spain I. L. Germanium at high pressures. Phys. Rev. B 34, 362–368 (1986). - PubMed
    1. Nelmes R. J. et al.. Imma phase of germanium at ∼80 GPa. Phys. Rev. B 53, R2907–R2909 (R) (1996). - PubMed

Publication types

LinkOut - more resources