Pressure induced wurtzite-to-zinc blende phase transition in ZnO at finite temperature
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Feng Liu Department of Materials Science and Engineering, University of Utah, Salt Lake City, Utah 84112 (Received 21 June 2008; accepted 21 August 2008)
We predict a possible phase transition of ZnO from wurtzite to zinc blende structure using first-principles molecular-dynamics simulations. By calculating the Gibbs free energies of the two phases as a function of temperature and hydrostatic pressure, we show that their energy difference decreases continuously with increasing temperature and pressure, and the vibrational entropy plays an important role on the location of the phase transition point. At 300 K, the phase transition is expected to occur at a pressure lower than 30 GPa with an activation energy barrier of 0.386 eV/atom. The transition path was also simulated, along which the system goes through a transient face-centered orthorhombic structure to overcome the energy barrier. Our theory results may be valuable for stabilizating the zinc blende ZnO in experiment.
I. INTRODUCTION
Zinc oxide (ZnO) has received worldwide attention due to its many interesting electrical, optical, and magnetic properties.1,2 As one of the prototypical functional materials, it is characterized by the wide band gap (3.37 eV) and large excitation binding energy (60 meV) which have made it very promising for ultraviolet emitting devices, transparent conductors, and so on.1,3,4 Bulk ZnO is known to crystallize only in the hexagonal wurtzite (WZ, P63mc space group) phase under ordinary conditions because of its ionicity that resides at the borderline between the covalent and the ionic materials.5 However, this thermodynamically stable phase has some drawbacks, such as spontaneous polarization and p-type doping difficulty, which have posed a challenge for the exploitation of ZnO optoelectronic devices and the improvement of their performance.1,3,5 Furthermore, the crystal structure difference has to be solved when constructing the heterostructures with the zinc blende (ZB, F4¯3m) phase II-VI materials.5 On the other hand, the ZB phase of II-VI semiconductors has shown superior properties in terms of lower carrier scattering, higher crystallographic symmetry, higher saturated electron drift velocity, and higher doping efficiencies.5 For these reasons, the metastable
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2008.0410 J. Mater. Res., Vol. 23, No. 12, Dec 2008
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ZB ZnO is of wide interest and is expected to solve certain problems associated with the WZ ZnO. The stability of crystallographic phases is highly related to the extrinsic factors. For the requirement of stability preferences, appropriate conditions, such as high temperature,6 high pressure,7,8 and heavy dopant,9 are necessary for forming a metastable structure. Bates et al. discovered the rocksalt (RS, Fm3¯m) phase ZnO under high pressure for the first time.10 From then on, much work has been carried out on the WZ-to-RS transition with notable progress.7,11 However, little
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