Abstract
Spontaneous and piezoelectric polarization in the nitrides is analyzed. The slab model was designed and proved to be appropriate to obtain the spontaneous polarization in AlN, GaN and InN. The spontaneous polarization and polarization related electric fields in AlN, GaN and InN were determined using DFT slab calculations. The procedure generates single value of spontaneous polarization in the nitrides. It was shown that Berry phase polarization may be applied to determination of spontaneous polarization by appropriate addition of polarization induced electric fields. The electric fields obtained from slab model are consistent with the Berry phase results of Bernardini et al. The obtained spontaneous polarization values are: 8.69*10-3 C/m2, 1.88*10-3 C/m2, and 1.96*10-3 C/m2 for AlN, GaN and InN respectively. The related Berry phase polarization values are 8.69*10-2 C/m2, 1.92*10-2 C/m2, and 2.86*10-2 C/m2, for these three compounds, respectively. The GaN/AlN multiquantum wells (MQWs) were simulated using ab intio calculations. The obtained electric fields are in good agreement with those derived from bulk polarization values. GaN/AlN MQWs structures, obtained by MBE growth were characterized by TEM and X-ray measurements. Time dependent photoluminescence measurements were used to determine optical transition energies in these structures. The PL obtained energies are in good agreement with ab initio data confirming overall agreement between theoretical and experimental data.
Highlights
Macroscopic polarization of crystalline materials is a physically important property of infinite systems such as solids[1,2] and finite systems including molecules and nanoobjetcs.[3]
The ab intio simulations are directly applied to GaN/AlN multiquantum wells (MQWs) structure, and the derived electric fields are compared with those obtained from spontaneous polarization formalism
Optimal approach was proposed by Ferreira et al.,[50] in two variants known as localdensity approximation or generalized-gradient approximation half-electron technique (LDA-1/2 and Generalized Gradient Approximation (GGA)-1/2), which approximately includes self-energies of excitations in semiconductors, providing band gap energies, effective masses, and band structures in good agreement with experimental values.[51]
Summary
The built-in electric fields affect energies of quantum states that is known as Stark effect. The electric field in quantum structures changes energy of quantum states, giving rise to phenomenon known as Quantum Confined Stark Effect (QCSE).[17] In addition, the strain induced fields may contribute to this effect significantly, especially in strained quantum low dimensional structures frequently used in modern devices that are built on polar nitrides (0001) surface.[17,18,19,20]. The approach based on direct slab ab initio simulations is used to obtain spontaneous polarization for bulk wurtzite AlN, GaN and InN. The ab intio simulations are directly applied to GaN/AlN MQWs structure, and the derived electric fields are compared with those obtained from spontaneous polarization formalism. The results of theoretical calculations and experimental data are compared and the conclusions are drawn
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