Stacking Faults Reveal Spontaneous Polarization of Wurtzite Semiconductors

The spontaneous polarization is a fundamental property of wurtzite semiconductors or any other pyroelectric crystal. It results in electric fields in semiconductor heterostructures affecting the design of devices for solid-state lighting or power electronics. However, this quantity is difficult to directly assess experimentally as it manifests itself only at interfaces and, to complicate things, is usually superimposed by the piezoelectric polarization due to misfit-related strain at the interface.

Figure 1 False-color monochromatic cathodoluminescence maps superimposed on the corresponding cross-sectional scanning electron micrograph of a GaN microcrystal. The detected energies are color coded. The cathodoluminescence intensity related to stacking faults shows a characteristic elongation along the basal plane, while the upper part of the crystal exhibits luminescence at the energy known for strain-free bulk material.

Stacking faults are local zinc-blende (cubic close-packed  lattice) modifications in a wurtzite matrix (hexagonal  close-packed lattice) and can be considered as quantum wells  due to a difference in the energy gaps of the two phases and  a band-offset between the two phases. Cathodoluminescence  spectroscopy and imaging in a scanning electron microscope is  used to assess the optical emission of such quantum wells of  different thickness, i.e. different stacking fault types, on the cross-sections of GaN microcrystals. 

 

The spontaneous polarization of the zinc-blende phase vanishes  as a result of the higher symmetry of this phase, and the misfit  strain at the wurtzite/zinc-blende interface is negligible.  Therefore, stacking faults are an ideal system to study the  spontaneous polarization of the wurtzite phase. Using  Poisson-Schrödinger calculations and density functional theory,  it is shown that the change in the emission energy as a function of thickness is governed by the spontaneous polarization, thereby allowing for a deduction of an experimental value for the spontaneous polarization  of GaN by treating the stacking faults as plate capacitors. Our obtained value for the spontaneous polarization of GaN is (-0.022+/-0.007) C/m². This approach should be applicable also to  other important wurtzite materials such as SiC and ZnO.

Publication
1 Autor J. Lähnemann , O. Brandt , U. Jahn , C. Pfüller , C. Roder , P. Dogan , F. Grosse , A. Belabbes , F. Bechstedt , A. Trampert , L. Geelhaar
Titel

Direct experimental determination of the spontaneous polarization of GaN

Source Phys. Rev. B , 86 , 081302(R) ( 2012 )
DOI : 10.1103/PhysRevB.86.081302 | Download: PDF | 2267 Cite : Bibtex RIS
J. Lähnemann, O. Brandt, U. Jahn, C. Pfüller, C. Roder, P. Dogan, F. Grosse, A. Belabbes, F. Bechstedt, A. Trampert, and L. Geelhaar