Laplace correction was used for solutions of Poisson equation in density functional theory (DFT) slab simulations of GaN (0001) surface. In accordance to the scenario of Meyer and Marx [Phys. Rev. B 67, 35403 (2003)], the electric field, emerging within the slab, is determined by the condition that Fermi energy is equal to the energies of valence band maximum and conduction band minimum at both sides of the slab. In contrast to their predictions that the field is identical for different slab thicknesses, it was found from DFT calculations that the electric potential difference between two sides of the slab is kept constant. Therefore, the energy of the slab does not diverge for large slab thickness. It was also found that, in most cases, the field in the slab is determined by pinning by surface states. Thus the change in opposite side slab termination, both by different types of atoms and by their location, can be used to change electric field in the slab, creating a tool that can be used to simulate the change in surface properties due to doping of the bulk. It was shown that, depending on the electric field, the energy of surface states changes in the way different from the band states. This change could be large, comparable to the bandgap. Despite the fact that the Fermi energy can be pinned to surface states, it may change its energy with respect to valence band and conduction band as a function of the doping of bulk semiconductor. These results are in agreement with the experimental data showing variation in Fermi energy at the GaN (0001) surface in function of the doping in the bulk. It was also shown using the electron density and potential profiles that ten Ga–N atomic layers are necessary to achieve small quantum overlap of the surface states of both sides of the slab, which is required for high precision simulations of the surface.

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.

In this paper, simulations of I-V characteristics and band structures of magnesium and silicon doped gallium nitride diodes are presented. The numerical algorithm is based on the drift-diffusion semi-classic model, with the van Roosbroeck differential equation system involved. The model accounts for trap-assisted tunneling, which provides better agreement between the predicted and experimental I-V characteristics of p-n junctions in the low-bias range. We have performed one-dimensional simulations of devices. We compare the results with the results obtained from the standard drift-diffusion model. It is shown that taking the trap-assisted tunneling into account leads to good agreement with experimental data. We also demonstrate that a high doping of the p-n junctions may significantly increase the nonradiative recombination rate due to the prescribed effect.

Ab initio simulations were used to determine electron affinity, work function, and ionization energy of AlN, GaN, and InN polar surfaces. The work function depends weakly on the doping in the bulk for the nitrides due to pinning of Fermi level at all polar surfaces. At the metal surface, it is smaller, equal to 3.87, 4.06, and 2.99 eV for AlN, GaN, and InN, respectively, while at the nitrogen side, it is much higher: 9.14, 9.02, and 8.24 eV. It was shown that the electron affinity and ionization potential do not obey the bandgap rule because of the quantum overlap repulsion of the surface and band states: conduction at the metal, and valence at the nitrogen side. The shift is substantial, even more than 1 eV, which may explain the first measured InN identified bandgap of 1.9 eV and the later much lower value of 0.7 eV. Cesium at both polar GaN surfaces does not create bonding states, nevertheless initially decreases electron affinity by charge transfer to surface states reducing electric dipole layer so that at some point the electron affinity becomes negative. At the Ga side, the positively charged Cs ions reduce the energy of Cs 6s states down to the Fermi level at about 0.3 monolayer (ML) coverage, the ionization of additional Cs adatoms is terminated, the electron affinity increases to saturate at 0.75 ML coverage. At the N-side, the Fermi level is pinned by N-broken bond states located close to the valence band maximum. At 0.75 ML Cs coverage, the nitrogen states are all filled, the additional Cs adatoms have to keep their electrons, and the Fermi level jumps from the N-state to the Cs 6s state in the bandgap, which drastically changes the work function. Additional Cs adatoms are not ionized; therefore, the electron affinity is steeply increasing.

It is shown that charge transfer, the process analogous to formation of semiconductor p-n junction, contributes significantly to adsorption energy at semiconductor surfaces. For the processes without the charge transfer, such as molecular adsorption of closed shell systems, the adsorption energy is determined by the bonding only. In the case involving charge transfer, such as open shell systems like metal atoms or the dissociating molecules, the energy attains different value for the Fermi level differently pinned. The Density Functional Theory (DFT) simulation of species adsorption at different surfaces, such as SiC(0001) or GaN(0001) confirms these predictions: the molecular adsorption is independent on the coverage, while the dissociative process adsorption energy varies by several electronvolts.

The results of comprehensive theoretical and experimental study of binary GaN/AlN multi-quantum well (MQW) systems oriented along polar c-direction of their wurtzite structure are presented. A series of structures with quantum wells and barriers of various thicknesses were grown by plasma-assisted molecular-beam epitaxy and characterized by x-ray diffraction and transmission electron microscopy. It was shown that in general the structures of good quality were obtained, with the defect density decreasing with increasing quantum well thickness. The optical transition energies in these structures were investigated comparing experimental measurements with ab initio calculations of the entire GaN/AlN MQW structure depending on the QW widths and strains, allowing for direct determination of the energies of optical transitions and the electric fields in wells/barriers by electric potential double averaging procedure. Photoluminescence (PL) measurements revealed that the emission efficiency as well as the shape of luminescence spectra correlated well with their structural quality. Additionally, due to the Quantum-Confined Stark Effect, the emission energy decreased by over 1 eV for quantum well thicknesses increasing from 1 nm up to 6 nm, and this effect was accompanied by the drastic drop of the PL efficiency. The experimental results are consistent with theoretical models. Comparison of experimental data obtained by a number of different characterization techniques with the density functional theory results received on the same geometry structure allowed to prove directly the theoretical models and to determine the polarization and the oscillator strengths in the AlN/GaN nitride systems for the first time.