Recent progress in the investigation of the role of charge on semiconductor surfaces has been reviewed. The review begins with contributions to the calculations and data analysis. This new procedure includes the application of the Laplace correction method in ab initio calculations. The new analysis incorporates the projected density of states (PDOS) and Crystal Orbital Hamilton Population (COHP) and averaging of the electric potential to derive its smoothed long-range variation in space, parallel to plots of real-space band profiles. These methods include the discovery of spurious Coulomb interactions between the separated subsystems, which have different Fermi levels. The ab initio use of a single Fermi level may cause incorrect electron redistribution, an artificial charge of the separated subsystems, and spurious interactions. The quantum nature of the charge influence on semiconductor surfaces stems from the delocalization of electrons, which leads to the emergence of an external surface dipole, which is important for determining the workfunction and plays a role in the proposed thermalization of the adsorbate via electron tunnelling. The kinetic energy loss of the adsorbate (i.e., its thermalization) occurs via the tunnelling of electrons into the solid interior owing to the strong external dipole electric field. The other charge-related quantum effect is related to the known subsurface dipole charge layer. New simulations of the variation of the electric potential within a slab model show the band bending at the semiconductor surfaces that induces the Surface States Stark Effect (SSSE) and misrepresents the surface band diagrams. This underlines the role of pinning the Fermi level and its connection to subsurface dipoles. The charge balance determines the occupation of the surface states and the symmetry and periodicity of surface reconstructions. The occupation of the surface states may be changed by adsorption, both by the new donated electrons and by the emergence of new quantum states. As their numbers could be different, this leads to a jump in the Fermi level pinning and adsorption energy at selected critical coverages. Thus, the Fermi level becomes free, subsurface dipoles disappear, and bands become flat. The adsorption energy jump may reach several electronvolts, which may change the adsorbate equilibrium vapor pressure by several orders of magnitude. Such a flat-band state is likely to occur during the growth of crystals that fall within such pressure intervals. Additionally, quantum effects may include resonant bonding involving several states, which leads to fractional occupation. The existence of resonant states resolves the existing inconsistencies between bonding and lattice symmetry in nitrides. This effect is observed not only at the stability points but also at the activated complex position in the diffusion jumps. Additionally, the effects include the quantum state energy increase during the jump, shifting them to the vicinity of the Fermi level or even above. According to quantum statistics, the state occupation is reduced, which affects the energy barrier for diffusion. Thus, these results demonstrate the role of quantum effects in the charge control of semiconductor surfaces.

Optical absorption and reabsorption of light emitted from the active regions in nitride laser diodes (LDs) reduce the light extraction efficiency of these devices. The presence of Si and Mg can considerably increase optical absorption, an effect that is much stronger in the high-energy (short-wavelength) spectral range. This absorption increase is governed by the ionization of Si donor and Mg acceptor states, which are controlled by the electron and hole quasi-Fermi levels. We demonstrate that absorption increases due to enhanced Mg ionization caused by compensation in the p-type region, alongside high Si ionization in the n-type region. A theoretical explanation is provided for the observed increase in optical efficiency achieved by removing dopants from the waveguides. Furthermore, superior material quality is shown to yield low absorption levels, especially within the Mg-doped p-type region of the device.

Ab initio calculations were used to disentangle the mystery of Nakamura activation of p-type in Mg doped MOVPE grown gallium nitride, the key process leading to the 2014 Nobel Prize in Physics. Calculations were used to obtain the equilibrium state of the hydrogen atom deep in the GaN bulk and at the GaN(0001) surface. It was shown that the H position within bulk GaN depends on the Fermi level: in n-type GaN, it is located in the channel, whereas in p-type GaN, it is attached to the N atom, breaking one of the GaN bonds. In contrast, at the GaN(0001) surface, H is attached in the on-top position for any hydrogen coverage; for low and high H-coverage, the Fermi level is pinned at the Ga - broken bond state and at the valence band maximum (VBM), respectively. The diffusion path from the bulk to the surface was obtained when the Fermi level was high and low, the barrier was zero, and ΔE_bar≈ 1.717 eV, which effectively blocked hydrogen escape into the vapor. Thus, high H coverage, that is, high hydrogen pressure in the vapor, prevents H from escaping from the bulk to the surface, whereas at low coverage (low hydrogen pressure), the process is barrierless. It is therefore proven that the hydrogen escape control step in the Nakamura process is the transition of hydrogen from the bulk to the surface, which is controlled by the position of the Fermi level at the surface. Molecular hydrogen desorption from the surface is easy for high H coverage and difficult for low, thus opposite to observed experimentally thus this process is not the determining step in activation. A full thermodynamic estimate of the maximal partial pressure of hydrogen in the vapor, corresponding to the transition of the Fermi level from the Ga-broken bond state to the VBM, was used to establish the maximal hydrogen pressure limit for the p-type Mg activation process.

Electrical properties of contact to p-type nitride semiconductor devices, based on gallium nitride were simulated by ab initio and by drift-diffusion calculations. The contact electric properties are shown to be dominated by electron transfer form metal to GaN related to Fermi level difference both by ab initio and model calculation. The results indicate on high potential barrier for holes leading to nonohmic character of the contact. The electrical nature of the Ni-Au contact formed by annealing in oxygen atmosphere is elucidated. The doping influence on the potential profile in p-type GaN was calculated by in drift-diffusion model. The energy barrier height and width for hole transport is determined. Based on these results, new type of the contact, is proposed. The contact is created employing multiple layer implantation of the deep acceptors. The implementation of such design promise to attain superior characteristics (resistance) as compared to other contacts used in bipolar nitride semiconductor devices. The development of such contact will remove one of the main obstacles in the development of highly efficient nitride optoelectronic devices both LEDs and LDs: energy loss and the excessive heat production close to the multiple quantum wells system

The emission properties of GaN/AlN and Al0.08Ga0.92N/AlN polar multi-quantum wells (MQWs) have been investigated by photoluminescence (PL) and time-resolved PL (TRPL) measurements as a function of temperature and pressure. The results were correlated with ab initio calculations. The samples were grown by plasma-assisted molecular-beam epitaxy. Their structural/chemical features were characterised by X-ray diffraction (XRD), scanning transmission electron microscopy (STEM), and secondary ion mass spectrometry (SIMS). The well/ barrier equal widths in the series of samples varied from 1.5 to 5 nm for GaN/AlN and from 2 to 4 nm for AlGaN/ AlN MQWs, respectively. Due to the strong internal electric fields, both types of MQWs revealed some similarities in their optical properties: a redshift of the PL with increasing well width, a strong increase of the PL decay times, and a decrease of the pressure coefficients for wider wells. However, some differences were also observed: the emission energies of AlGaN/AlN structures were higher due to the presence of aluminium in the wells, activation energies were lower due to lower band offset, and internal quantum efficiencies and pressure coefficients were higher because of smaller lattice mismatch in AlGaN/AlN (weaker piezoelectric effects). The PL energies, oscillator strengths, and PL pressure dependencies were determined by density functional theory calculations, obtaining reasonable agreement with experimental measurements. Our analysis demonstrated a significant influence of piezoelectric effects on the emission properties of nitride-based QWs, and revealed that nonlinear processes are more important in the AlGaN/AlN system than in GaN/AlN, pointing to the presence of an additional mechanism of optical emission in AlGaN alloys.

It has been demonstrated that quantum effects play an important role in nitrogen adatom diffusion over partially Ga-covered GaN(0001) surfaces. This is partially related to the bonding in both the initial and activated complex states. It is shown that wurtzite gallium nitride is bonded differently from standard semiconductors having two separate valence subbands: upper by gallium 4sp3 hybridized orbitals and nitrogen resonant 2p states, and lower by gallium 3d and nitrogen 2s orbitals. Second, the diffusion energy barrier may be changed owing to the quantum statistics of electrons governed by the Fermi energy, as shown in the case of nitrogen diffusion over a clean and gallium-covered Ga-terminated GaN(0001) surface. Under fractional Ga coverage of the GaN(0001) surface, the nitrogen diffusion energy barrier is at the saddle point. The barrier affects the electron redistribution between the surface quantum states at both the initial and saddle points. In the case of full GaN coverage, the diffusion path is from the top N adatom configuration to the H3 site, which corresponds to the maximal energy. Therefore the diffusion barrier is ΔEbar = 1.18 eV for clean and ΔEbar = 0.92 eV for (1/6) ML to finally ΔEbar = 1.23 eV for full Ga coverage. Thus, the overall barrier was ΔEbar = 0.92 eV. The identified stable N-on-top configuration for Ga coverage is essential for the GaN growth mechanism.

In this study, the fundamental properties of spontaneous and piezo polarization and surface polarity were defined. It was demonstrated that the Landau definition of polarization as a dipole density could be used in infinite systems. Differences between bulk polarization and surface polarity were distinguished, thus creating a clear identification of both components. This identification is in agreement with numerous experimental data-red shift presence and absence for wurtzite and zinc blende multiquantum wells (MQWs), respectively. A local model of spontaneous polarization was created and used to calculate spontaneous polarization as electric dipole density. The proposed local model correctly predicted the c-axis spontaneous polarization values of nitride wurtzite semiconductors. In addition, the model's results are in accordance with a polarization equal to zero for the zinc blende lattice. The spontaneous polarization values obtained for all wurtzite III nitrides are in basic agreement with earlier calculations using the Berry phase. Ab initio calculations of wurtzite nitride superlattices in Heyd-Scuseria-Ernzerhof (HSE) approximation were performed to derive polarization-induced fields in coherently strained lattices, showing good agreement with the polarization values. Strained superlattice data were used to determine the piezoelectric parameters of wurtzite nitrides, obtaining values that are in basic agreement with earlier data. Zinc blende superlattices were also modeled using ab initio HSE calculations, showing results that are in agreement with the absence of polarization in all nitrides in zinc blende symmetry.

Recent measurements of the band properties of AlN and GaN by fluorescence yield absorption and soft X-ray emission spectroscopies revealed that their valence band (VB) is composed of two separate subbands. The upper VB subband of GaN is composed of gallium sp and nitrogen p orbitals; the lower subband consists of metal d and nitrogen s orbitals. These findings were confirmed by extensive ab initio simulations. These results are not consistent with the standard tetrahedrally coordinated semiconductors, which are bonded by sp3-hybridized orbitals of metal and nonmetal atoms. The new analysis techniques and ab initio simulations create a new picture, allowing the calculation of overlap integrals to determine the bond order in these crystals. According to these results, bonding occurs between resonant p-states of nitrogen and sp3-hybridized metal orbitals in tetrahedral nitrides, allowing tetrahedral symmetry to be maintained. A similar resonant bonding mechanism is observed in hexagonal BN, where the p orbitals of nitrogen create three resonant states necessary for maintaining the planar symmetry of the lattice. In addition, nonresonant π-type bonds in BN are created by the overlap of pz orbitals of boron and nitrogen. BN bonding differs from that in graphene, where carbon states are fully sp2-hybridized. Additionally, π-type bonds in graphene have no ionic contributions, which leads to the formation of Dirac states with linear dispersion close to the K point, closing the band gap.

Single crystals of hexagonal BN have been grown from solutions on the surface of Ni-Cr and pure Ni metals under a pressure of N2 gas of 1000-1500 bar. The transparent and colorless hBN crystals obtained in this study exhibited high structural quality and uniformity as confirmed by micro-Raman mapping, showing less than 8 cm-1 widths of the high-frequency E2g Raman peaks corresponding to the intralayer vibrations of the B and N atoms in hBN. Optical absorption measurements indicated the indirect character of electronic transitions in bulk hBN. Due to the solubility of nitrogen increased by a factor of 30-40 by compression, it was possible to grow significantly thicker (up to 30 μm) hBN crystals from pure Ni solvent than by a similar approach at atmospheric pressure. The addition of chromium to the solution seriously disrupted the hBN crystallization process, as at elevated N2 pressure efficient synthesis of CrN occurred at the same time. The results of the crystallization experiments are discussed in the context of the thermodynamic properties of III-N compounds and the effect of the metal solvent on the conditions of thermal stability of BN concerning its components. To further assess the quality of the crystals and evaluate their potential for applications, thin flakes exfoliated from the grown hBN crystals were applied in graphene-based devices. Remarkably high carrier mobility, exceeding 21.2 m2·V-1·s-1 at 230 K for both electrons and holes, was observed in magnetotransport studies on hBN-encapsulated graphene transistors.

The electrical properties of contacts to p-type nitride semiconductor devices, based on gallium nitride, were simulated by ab initio and drift-diffusion calculations. The electrical properties of the contact are shown to be dominated by the electron-transfer process from the metal to GaN, which is related to the Fermi-level difference, as determined by both ab initio and model calculations. The results indicate a high potential barrier for holes, leading to the non-Ohmic character of the contact. The electrical nature of the Ni–Au contact formed by annealing in an oxygen atmosphere was elucidated. The influence of doping on the potential profile of p-type GaN was calculated using the drift-diffusion model. The energy-barrier height and width for hole transport were determined. Based on these results, a new type of contact is proposed. The contact is created by employing multiple-layer implantation of deep acceptors. The implementation of such a design promises to attain superior characteristics (resistance) compared with other contacts used in bipolar nitride semiconductor devices. The development of such contacts will remove one of the main obstacles in the development of highly efficient nitride optoelectronic devices, both LEDs and LDs: energy loss and excessive heat production close to the multiple-quantum-well system.

Elastic isotropy is a phenomenon in which a material responds uniformly to stress, regardless of its direction. In the case of cubic crystals, which possess distinct crystallographic directions, this represents a remarkable manifestation of quantum mechanics in macroscopic objects. Such behavior of a crystal cannot be explained within the framework of classical physics. The phenomenon is closely related to the balancing of internal forces resulting from Coulomb interactions, Pauli repulsion, and the overlap in the bands when stress is applied to the crystal. On the macroscopic level, this corresponds to the relationship between elastic constants given by 2 C44/(C11 - C12) = 1. The subject of the present work is to demonstrate the influence of the number of valence electrons per atom in binary titanium alloys with vanadium, niobium, and tantalum on the shape of the anisotropy curve. The result of the work is the identification of a new Ti-53Nb alloy exhibiting elastic isotropy, and the demonstration that this phenomenon cannot occur for TiTa alloys, in the range of mechanical stability of these alloys. This study includes a summary of the main trends exhibited by the elastic constants, Young's modulus, and bulk modulus of the discussed Ti-based alloys, based on ab initio methods. Additionally, the work addresses the well-known difficulty in determining the elastic constants of vanadium and niobium, along with a proposed solution that offers significant improvement in reproducing experimental results compared to the conventional use of the PBE (Perdew-Burke-Ernzerhof) functional.

It is shown that quantum effects play determining role in nitrogen adatom diffusion due to several different factors. This could be related to the change of the energy of the quantum states and also due to the redistribution of electrons between the quantum states, both full and resonant, via quantum statistics partially governed by the Fermi energy level. These effects were studied in the case of nitrogen diffusion over clean and gallium covered Ga-terminated GaN(0001) surface. For the fractional coverage the density functional theory (DFT) calculations show that at the saddle point configuration the redistribution of electrons between different quantum states may affect the surface diffusion barrier significantly. The other quantum influence occurs via the change of the minimal energy configuration. Under fractional Ga coverage of GaN(0001) surface the nitrogen diffusion energy barrier proceeds from the resonant states governed energy minimal H3 site across the saddle point in the bridge configuration. At this path the barrier is affected the electron redistribution between surface quantum states both in the initial and the saddle point. In the case of the full GaN coverage the diffusion path is from on-top N adatom configuration via H3 site that corresponds to maximal energy. Therefore the diffusion barrier is Ebar= 1.18 eV for clean and Ebar= 0.92 eV for (1/6) ML to finally Ebar= 1.23 eV for full Ga coverage. Thus the overall barrier is reduced to Ebar= 0.92 eV due to quantum statistics effects. The identified stable N on-top configuration for the full coverage is essential for atomic mechanism of GaN growth in Ga-rich regime.

Ab initio calculations were applied to large size slabs simulations for determination of the properties of the principal structures of the stoichiometric GaN(0001) surface. The results are different from published previously: stoichiometric GaN(0001) surface structure is characterized by mixed structure in which 3/8 top layer Ga atoms remain in standard position with $sp^3$ hybridized bonding while the remaining 5/8 top layer Ga atoms is located in plane of N atoms with $sp^2$ hybridized bonding. This involves charge transfer to the previous one, entailing energy optimization, therefore Ga-s and Ga-p energy difference is driving force for the transition. In large size surfaces Ga atoms create (4 x 4) reconstruction which is additionally stabilized by strain optimization. Heavy doping in the bulk changes $sp^3$ to $sp^2$ ratio thus confirming the charge transfer mechanism of the reconstruction. The charge transfer energy optimization is universal for all surfaces terminated by $sp^3$ bonded atoms.

Combined cathodoluminescence (CL) and photoluminescence (PL) studies in time resolved regime were used to characterize recombination properties of the semiconductor material. The characterization is based on new instantaneous decay time analysis [P. Strak et al., Journal of Alloys and Compounds 823, 153791 (2020)]. The investigation provided detailed picture of the deexcitation processes in GaN bulk layer close to the surface. The studies, based on different type of excitation allowed to identify the difference in various recombination modes related to the presence of the semiconductor surface. Moreover, it is demonstrated that the method used allows for recombination analysis of local sub-surface semiconductor material region (CL) as well as large bulk volume (PL).

Extensive ab initio density functional theory molecular dynamics calculations were used to evaluate stability conditions for relevant phases of InN. In particular, the p-T conditions of the thermal decomposition of InN and pressure-induced wurtzite–rocksalt solid–solid phase transition were established. The comparison of the simulation results with the available experimental data allowed for a critical evaluation of the capabilities and limitations of the proposed simulation method. It is shown that ab initio molecular dynamics can be used as an efficient tool for simulations of phase transformations of InN, including solid–solid structural transition and thermal decomposition with formation of N2 molecules. It is of high interest, because InN is an important component of epitaxial quantum structures, but it has not been obtained as a bulk single crystal. This makes it difficult to determine its basic physical properties to develop new applications.

On semiconductor growth surfaces, surface reconstructions appear. Estimation of the reconstructed structures is essential for understanding and controlling growth phenomena. In this study, the stability of a mixture of two different surface reconstructions is investigated. Since the number of candidate structures is enormous, the structures sampled by Bayesian optimization are analyzed. As a result, the local electron counting (EC) rule alone was found to be insufficient to explain such stability. Then, augmenting the EC rule, a data-driven Ising model is proposed. The model allows the evaluation of the whole enormous number of candidate structures. The approach is expected to be useful for theoretical studies of such mixtures on various semiconductor surfaces.

Defect-mediated nonradiative recombination, known as Shockley-Read-Hall (SRH) recombination is reformulated. The introduced model considers Coulomb attraction between charged deep defect and the approaching free carrier, showing that this effect may cause considerable increase of the carrier velocity approaching the recombination center. The effect considerably increases the carrier capture rates. It is demonstrated that in the typical semiconductor device or semiconductor medium, the SRH recombination cannot be neglected at low temperatures. The SRH is more effective in the case of low doped semiconductors. Effective screening by mobile carrier density could reduce the effect, leading to SRH rate increase.