Critical Evaluation of Various Spontaneous Polarization Models and Induced Electric Fields in III-Nitride Multi-Quantum Wells.

Ashfaq Ahmad,Stanislaw Krukowski,Pawel Kempisty,Pawel Strak,Aleksandra Wierzbicka,Agata Kaminska,Konrad Sakowski,Izabella Grzegory,Jacek Piechota,Serhii Kryvyi,Kamil Koronski,Eva Monroy

doi:10.3390/ma14174935

Abstract

In this paper, ab initio calculations are used to determine polarization difference in zinc blende (ZB), hexagonal (H) and wurtzite (WZ) AlN-GaN and GaN-InN superlattices. It is shown that a polarization difference exists between WZ nitride compounds, while for H and ZB lattices the results are consistent with zero polarization difference. It is therefore proven that the difference in Berry phase spontaneous polarization for bulk nitrides (AlN, GaN and InN) obtained by Bernardini et al. and Dreyer et al. was not caused by the different reference phase. These models provided absolute values of the polarization that differed by more than one order of magnitude for the same material, but they provided similar polarization differences between binary compounds, which agree also with our ab initio calculations. In multi-quantum wells (MQWs), the electric fields are generated by the well-barrier polarization difference; hence, the calculated electric fields are similar for the three models, both for GaN/AlN and InN/GaN structures. Including piezoelectric effect, which can account for 50% of the total polarization difference, these theoretical data are in satisfactory agreement with photoluminescence measurements in GaN/AlN MQWs. Therefore, the three models considered above are equivalent in the treatment of III-nitride MQWs and can be equally used for the description of the electric properties of active layers in nitride-based optoelectronic devices.

Highlights

Spontaneous polarization has attracted considerable attention in recent decades
The presence of spontaneous polarization can induce an electric field in bulk insulators having small size
The Fermi level is located in the band gap, far away from the conduction and valence bands, and the density of mobile charge is negligible, so that the field is not screened by free carriers

Summary

Introduction

Despite its relatively early identification by Landau and Lifsic, the nature of this phenomenon was discussed by many authors [1,2,3,4] These investigations include Resta’s formulation of spontaneous polarization using the Berry phase, which allowed him to calculate the difference between two different configurations of a system [5]. The different III-nitride compounds have different lattice constants, so that the misfit strain is important in both heteroepitaxial layers and MQWs. The magnitude of the piezoelectric polarization can be comparable to the spontaneous polarization. From the point of view of applications, the value of the polarization in thick layers (tens of nanometers) is not relevant, since polarization-induced electric fields are screened out. The method of determination of the technologically relevant polarization differences and the resulting electric fields in the wells and barriers will be described

The Calculation Methods

Results

Ab-initio-calculated

The Experimental Verification